Either one would dominate off the bench. Milsap just doesn't have the size to be a starter, but he would dominate the second team as PF. AJ would be pretty good off the bench too. Put him in with Evans who can get all the backside rebounds when he shoots against triple teams. Favors needs to get experience as a starter if he is going to become elite. The other 2 have the experience, and unfortunately have shown that they aren't elite.
That being said, I think that Favors will come off the bench of both AJ and Milsap are on the team. I don't think they have the guts to bench AJ and they want to keep Milsap around, so they will tread lightly with him. I hope I'm wrong and there is a chance that I will be, but I don't see Favors starting until they get rid of one of the two.