One of the few here who uses both. 2D Classic for the real time action, 3D for the replays of goals.
I've been playing FM/CM for a fair few iterations and watching the match in 3D just doesn't 'feel' right to me, I also find that for me 2D classic makes it easier to notice tactical tweaks and the like. Don't really know why I view the goal replays in 3D, different perspective I guess.