I prefer AIO/2's over fitteds as I have a very wiggly bub and there is no way she would lay still enough to put two layers on. I have enough trouble getting one on and tend to do up the second side as she's rolling over or moving away.

I think they're a little trimmer then fitteds so fit under clothes better (not so necessary now, but during colder months). With Ai2's I still change completely each time, although I know that some only change the booster.