Like some of the records on them are very similar:
Empress Zhangsun: 後喜圖傳,視古善惡以自鑒,矜尚禮法。
"The empress liked books and biographies, examined the ancient good and evil as a mirror for herself, and was reverent of etiquette."
Empress Dugu: 后初亦柔順恭孝,不失婦道。
"The empress was originally also gentle, respectful, and filial, and did not lose female virtue."
Humility before becoming empress
Empress Zhangsun: "時隱太子釁鬩已構,後內盡孝事高祖,謹承諸妃,消釋嫌猜。"
"At the time Crown Prince Yin (Li Jiancheng)'s smears and quarrels had already sowed dissension. The empress in the inner palace strived to serve Emperor Gaozu (Li Yuan) filially, and respected and honoured his various concubines, clearing up suspicion"
Empress Dugu: "后姊為周明帝后,長女為周宣帝后,貴戚之盛,莫與為比,而后每謙卑自守,世以為賢。"
"The empress's sister was the empress of Ming of Zhou (Yuwen Yu), her eldest daughter was the wife of Xuan of Zhou (Yuwen Yun), in the flourishing of noble relatives, none could compare. Yet the empress was always humble and with solid moral character, and the people of the time regarded her as admirable."
Discussion of political matters
Empress Zhangsun: "長孫皇后性仁孝儉素,好讀書,常與上從容商略古事,因而獻替,裨益弘多。"
"Empress Zhangsun's character was benevolent, filial and frugal. She enjoyed reading, and often discussed matters of ancient times with the emperor, in order to present what was feasible and discard what was not, which was of great benefit."
Empress Dugu: "使宦官伺上,政有所失,隨則匡諫,多所弘益。" "後雅好讀書,識達今古,凡言事皆與上意合,宮中稱為二聖。"
"Empress Dugu sent eunuchs to wait upon the emperor. If the government had faults, she would thus rectify them, which was of great benefit."
"The empress always enjoyed reading, with knowledge of the present and past. Her words and deeds joined with the emperor's wishes, and the palace called the Two Sages."
(Empress Zhangsun was clearly a lot more discreet than Empress Dugu with her influence)
Empress Zhangsun: "性約素,服御取給則止。"
"The empress's character was frugal, taking what she needed and nothing else."
Empress Dugu: "後雅性儉約。帝常合止利藥,須胡粉一兩,宮內不用,求之竟不得。"
"The empress's character was always frugal. The emperor often prepared medicine to cure dysentery, which required one liang of foreign powder. The palace didn't have any, and after asking he still did not get it."
Showing awareness of the limits of female influence in politics
Empress Zhangsun: "與帝言,或及天下事,辭曰:「牝雞司晨,家之窮也,可乎?」帝固要之,訖不對。"
"She discussed with the emperor, and when a government matter came up, she would say 'When a woman interferes in politics, it finishes her family, how can I?'"
Empress Dugu: "有司奏曰:「《周禮》,百官之妻,命于王后。憲章在昔,請依古制。」後曰:「以婦人與政,或從此漸,不可開其源也。」不許。"
"The officials presented a memorial saying: 'In the Rites of Zhou, the wives of officials were conferred by the Queen. To comply with the laws of the past, we ask to follow the ancient system.' The empress said: 'The involvement of women in politics could permeate from this. This cannot become its origin.' Thus it was rejected."
Benevolence within the palace
Empress Zhangsun: "後廷有被罪者,必助帝怒請繩治,俟意解,徐為開治,終不令有冤;下嬪生豫章公主而死,後視如所生;媵侍疾病,輟所禦飲藥資之。下懷其仁。"
"When someone in the empress's court was to be punished, she would assist the emperor in his anger to ask for punishment, waiting for the anger to subside. Then she would slowly ask to lift the punishment, and in the end there would be no injustice. When Concubine Xia died giving birth to a daughter Princess Yuzhang, the empress raised her as her own, and if a concubine or maid was ill, she suspended her medical expenses to provide for her. This was the benevolence of her heart."
Empress Dugu: "後頗仁愛,每聞大理決囚,未嘗不流涕。", "後早失二親,常懷感慕,見公卿有父母者,每為致禮焉。"
"The empress was quite benevolent, and every time she heard that the Superintendent of Law Enforcement had executed a criminal, she would shed tears."
"The empress lost both her parents young, and she always thought of them fondly in her heart. When she saw a minister with a mother and father, she would always treat them with great respect."
Begging for the life of a brother who wronged them
Empress Zhangsun: "異母兄安業無行,父喪,逐後、無忌還外家。後貴,未嘗以為言。擢位將軍。後與李孝常等謀反,將誅,後叩頭曰:「安業罪死無赦。然向遇妾不以慈,戶知之;今論如法,人必謂妾釋憾於兄,無乃為帝累乎!」遂得減流越巂。"
"The empress's brother of a different mother, Anye, was unruly. After the death of their father, he chased out the empress and Wuji to live with their mother's family. When the empress became noble, she did not speak of this. He was selected as a general. Later he, Li Xiaochang and others plotted rebellion and were to be executed. The empress kowtowed and said: 'Anye's crimes deserve death without pardon. Yet he has not been kind of me, and the people know this; if it were according to the law, people would certainly say I unleashed revenge upon my brother, would this not be a burden to the emperor?' Thus his punishment was reduced to exile to the Yue region."
Empress Dugu: "后異母弟陀,以猫鬼巫蠱,咒詛於后,坐當死。后三日不食,為之請命曰:「陀若蠧政害民者,妾不敢言。今坐為妾身,敢請其命。」陀於是減死一等。"
"The empress's brother of a different brother, Dugu Tuo, used cat-spirit witchcraft to curse the empress. He was tried and sentenced to death. The empress refused to eat for three days, and begged for his life, saying: 'If Tuo were eroding the government and harming the people, I would not dare to speak. Now he is to be convicted for me, I dare to ask for his life.' Tuo's punishment was thus reduced by one degree."
Strictness with imperial relatives
Empress Zhangsun: "訓諸子,常以謙儉為先,太子乳母遂安夫人嘗白後,以東宮器用少,請奏益之。後不許,曰:「為太子,患在德不立,名不揚,何患無器用邪!」"
"When teaching her sons, the empress always put modesty and frugality first. The crown prince's wet-nurse Lady Sui'an once submitted a memorial to the empress, saying that the East Palace's materials were low, and asking to increase them. The empress refused, saying: 'A crown prince should only worry about not establishing virtue or not having fame. Why worry about not having materials?'"
(One thing about Empress Zhangsun I've noted is that the speech recorded in the text actually sounds quite assured, even if she has the habit of using the humble pronoun of 妾. She has a tendency to use affirmative particles at the end of her sentences, or to use exclamations. Empress Dugu's speech reads similarly, though we have less speech from her)
Empress Dugu: 后每謂諸公主曰:「周家公主,類無婦德,失禮於舅姑,離薄人骨肉,此不順事,爾等當誡之。」
"The empress often said to the various princesses: 'The princesses of Zhou lacked in womanly virtue and were incestuous with aunts and uncles and distant from their flesh and blood. This is not a matter to follow, you all should observe this as a warning."
Refusing to put personal needs before that of the country
Empress Zhangsun: 太子言於後曰:「醫藥備盡而疾不瘳,請奏赦罪人及度人入道,庶獲冥福。」後曰:「死生有命,非智力所移。若為善有福,則吾不為惡;如其不然,妄求何益!赦者國之大事,不可數下。道、釋異端之教,蠹國病民,皆上素所不為,奈何以吾一婦人使上為所不為乎?必行汝言,吾不如速死!」太子不敢奏,私以語房玄齡,玄齡白上,上哀之,欲為之赦,後固止之。
"The crown prince said to the empress: 'We have prepared medicine to the fullest and yet your illness has not been cured. I ask to pardon criminals and allowing people to enter the Dao (Daoist temples), in order to obtain blessings.' The empress said: 'Life and death is fate, and there is no knowledge that can change it. If doing good deeds gives good fortune, then I have not done evil; if it does not, then what benefit is such an absurd request! A pardon is a major matter of the country, and cannot be frequent. Daoism and Buddhism are heretical doctrines, eroding the country and sickening the people, and such things would never be done for His Majesty. How could I, one woman, do something His Majesty would not? If it must be as you say, I would rather die quicker!' The crown prince did not dare to present, and secretly told it to Fang Xuanling. Fang Xuanling presented the memorial, and the emperor was mournful and wanted to give a pardon. The empress was stubborn in going against it, and stopped it from happening."
(Note that the language in the ZZTJ is very strong. The language in the JTS isn't as strong, but she sounds very self-assured there as well)
Empress Dugu: 大都督崔長仁,后之中外兄弟也,犯法當斬。高祖以后之故,欲免其罪。后曰:「國家之事,焉可顧私!」長仁竟坐死。
"The capital supervisor Cui Changren was the empress's maternal relative. He violated the law and was due to be executed. Gaozu (Yang Jian), because of the empress's friendship with him, wanted to pardon his crime. The empress said: 'This is a matter of the country, how could private interests be attended to!' Changren was thus tried and executed.'
Empress Zhangsun: "后所生長樂公主,太宗特所鐘愛,及將出降,敕所司資送倍於長公主。魏徵諫曰:「昔漢明帝時,將封皇子,帝曰:『朕子安得同於先帝子乎!』然謂長主者,良以尊於公主也,情雖有差,義無等別。若令公主之禮有過長主,理恐不可,願陛下思之。」太宗以其言退而告后,后嘆曰: 「嘗聞陛下重魏徵,殊未知其故。今聞其諫,實乃能以義制主之情,可謂正直社稷之臣矣。妾與陛下結髮為夫婦,曲蒙禮待,情義深重,每言必候顏色,尚不敢輕犯威嚴,況在臣下,情疏禮隔,故韓非為之說難,東方稱其不易,良有以也。忠言逆於耳而利於行,有國有家者急務,納之則俗寧,杜之則政亂,誠願陛下詳之,則天下幸甚。」
因請遣中使繼錢四百緡、絹四百匹以賜征,且語之曰:「聞公正直,乃今見之,故以相賞。公宜常秉此心,勿轉移也。」"
"The Princess Changle was due to be married. The emperor, because she was the empress's daughter, especially loved her, and decreed the officials to send a dowry double that of Princess Yongjia. Wei Zheng remonstrated, saying: 'In the past Emperor Ming of Han wanted to enfeoff the princes, saying: 'How can my sons compare to the sons of the previous emperor?' And for one who is older, who is respected above a princess, while there may be differences in feeling, etiquette does not have this distinction. If you were to order the princess's rites to go above that of a Grand Princess, rationally this cannot be done, and I hope Your Majesty considers this.' The emperor reported his words to the empress.
The empress sighed and said: 'I have heard Your Majesty values Wei Zheng, but I did not know why, now watching him lead in etiquette to suppress the feelings of the ruler, I now know he is an honest minister! Your Majesty and I have been married since we were youths, with great courtesy and deep feelings. Every word I say is prepared in advance, not daring to lightly violate your power; the situation for a subject with different rites and distant relationships, in the past Han Fei said this was hard, and Dongfang Shuo spoke of its unease, and it is very much true. Loyal words that violate the ears are beneficial for acting, in the matters of country and family, taking them will lead to a peaceful society, and resisting will lead to chaotic governance. I sincerely hope Your Majesty will take it, as the empire will be especially blessed.'
Thereupon she asked for Wei Zheng to be sent 400 coins and 400 bolts of silk, and said to him: 'I have heard of your uprightness, and now I see it, hence the reward. You should continue to uphold this heart, and do not change it.'"
(sidenote, but Wei Zheng remonstrances in a quite similar way to Empress Zhangsun. Both allude to historical examples to advise Li Shimin, but clearly Wei Zheng was more blunt in doing so)
Empress Dugu: "突厥嘗與中國交市,有明珠一篋,價值八百萬,幽州總管陰壽白后市之。后曰:「非我所須也。當今戎狄屢寇,將士罷勞,未若以八百萬分賞有功者。」百僚聞而畢賀。"
"The Turks once had a diplomatic exchange, and there was a case of pearls, worth eight million. The Manager of You Province, Yin Shou, asked for the empress to buy it. The empress said: 'This is not necessary for me. Now the Rong and Di are frequently invading, and the soldiers are exhausted. It is not as good as using the eight million to reward the meritorious.' The officials heard and congratulated her."
I think there are three areas where they notably diverge. The first is that Empress Dugu valued monogamous relationships, was jealous herself and valued people who remained faithful to their wives, while Empress Zhangsun was noted for not being jealous and did not care about monogamy. The second is that Empress Dugu was a lot more overt in her influence compared to Empress Zhangsun. And the third is that Empress Dugu was more prone to ruthlessness, such as her scheming against Gao Jiong, and while Empress Zhangsun was capable of ruthlessness (as shown by her involvement in the Xuanwu Gate Incident), she preferred to maintain good relationships with officials and asked for her husband to recall Fang Xuanling on her deathbed.
Outside of these areas, their political careers are relatively similar. They started out as wives of accomplished gentlemen who rose to power, and they became empress. During their time as empress they were noted for their frugality, benevolence within the palace, and used their influence for good (except at the end of Empress Dugu's influence with Yang Guang and Gao Jiong). While they did have influence, they also had to acknowledge the limits of female influence in the time period without creating trouble in the court, though Empress Dugu managed to be more overt than Empress Zhangsun. I guess it was personal preference; Empress Zhangsun seemed more introverted and adherent to protocol than Dugu Qieluo.