I would say that avoiding teachers you don't like is a high priority, maybe the highest. Having a teacher that will cause problems can sabotage the whole project.
That said, I have to say from experience that the subject acceleration with classroom changing will be highly dependent upon what he will miss while he's off in the other class. We had a problem with that this year with DS7 in 2nd grade and going to 3rd for reading and 5th for math. Eventually the system broke down when he got more and more upset at missing "specials" such as PE, music, art, while he was off at math or reading. The last month of school, we just left off the math move so that he could go to specials and not worry about it. Fortunately he's far enough ahead of even the 5th grade curriculum that he didn't really need it anyway.
I haven't any experience with having two different second-grade classes, but that seems like it might give one a feeling of not belonging anywhere in particular.
