I'm going to preface this theory with the fact that Mori is a deeply misunderstood character that the fandom mischaracterizes as being only (KEYWORD: ONLY) a pedophile instead of looking deeper. To my Mori fans out there, I see you, and I love you. (He personally isn't one of my favs, but I get it.) Now, I may also be interacting with the wrong side of fandom, but if 'Mori Ougai is not a Pedophile' is recognized as a registered tag on Ao3, then I'm certain many people feel that way.
I do not think Mori will choose Atsushi OR Dazai in his deal with Fukuzawa to gain an Agency member.
BSD is, at its core, a story about morals and finding yourself. How circumstances form the person you are and will become. (BEAST, from what I know, makes this even more clear.) And Mori, from what I have gathered, is well aware of this fact. (The circumstances part.) I am going to cut myself off here from going too into depth about what BSD truly is at its core, as I have many thoughts that can be for another day.
Continuing from where I left off, he is going to see how Atsushi's morals are more aligned towards saving others, and how Dazai has changed from the suicidal boy in the Mafia who ran away. Mori is, at the very least, going to make the best choice for the Mafia overall. That is why he took over as the Mafia's Boss. The entire point is that he looks at things clinically, and not emotionally, like the previous boss did.
If anything, heās going to look at the other members. And, in my opinion, Junichiro is likely the most qualified to join the Mafia out of everyone. He is the more violent of all of the Agency, willing to do anything after Naomi is injured. He's even willing to attempt to assassinate Mori during the Cannibalism Arc.
Ending this off with this; This is my opinion. I am open to hearing other's thoughts on who he may bring into the Mafia. Why it may be Dazai, Atsushi, or anyone else. Hell, I'm even willing to listen to if you think it'd be Fukuzawa, too!
I make my theories to speak with fandom, and hearing others' opinions is forever something I am open to.