Good question and good post. IMO I think AP and Shady are 2 totally different RBs. AP seems to hit the hole and go alot more than McCoy and he has great vision. McCoy sometimes dances a little too much, don't know how much difference it will effect him in 3 more years.
But AP is just one of those RBs that you really can't go wrong with, as far as McCoy having an off season, some of that is the Oline to blame. But he still tends to make silly decisions sometimes in which cost himself a lot of yards. So it really depends on the situation he is in if he is still a top RB in 3 more years, whether he is still in Buffalo or on some other team with a good OL.
I'll put this out there though, speaking for myself, I would rather have AP than McCoy if I had to choose. Not taking anything away fromMcCoy, both are great/good RBs, I just think AP seems to run better.