Do you think Sima You would have made a better Emperor than Sima Yan?
Probably.
Sima Yan’s biggest flaw, I think, was that he allowed factionalism to grow in his court. In particular, he let Jia Chong and his party grow too powerful and should have curbed them. His policy of allowing the various Princes to hold government authority was also dangerous and was what led to the War of Eight Princes. While it would have been a shame to lose the talents of men like Sima Fu, Sima Wang, Sima Rong, Sima Jun, and Sima You, obviously Sima Yan should have done something to prevent less talented, less ethical men (like Sima Lun) from gaining traction. Sima Yan’s third mistake, of course, was keeping Sima Zhong as the heir in spite of his condition.
Sima You probably would have done better on the first count. He kept himself out of factional conflicts, and he was not on good terms with Jia Chong. He probably would have limited Jia Chong’s power. The third count also would not have been much of an issue - Sima You’s heir would have been Sima Jiong, who was quite talented in his own right. I’m not sure what he would have done about the Princes, though, and that was an issue that really needed to be addressed.
As for other affairs, I think Sima Yan handled them well enough, and even if Sima You might have done better with Jin’s civil and military problems, I see no particular reason to criticize Sima Yan’s work in that area.
