Assuming health all around, AP is going to get a higher volume of touches. First, because that's the Norv way, second, because that's NOT the Reid way, and third, because Bell's guaranteed to miss a minimum of two games.
AP's also the best bet among the three to get a ton of goalline chances. Again, only because of Norv's history. His bellcows are notoriously prodigious at the GL -- it's just the way he runs the offense.
And then, even though he's not the pass catching threat Bell is, Norv once again steps to the plate, here. He's not really a switch-it-up, third-down-back kind of OC. His studs have always been around 50 catches a year, which won't be much less than Bell sees, given the two-game delay in his production.
All in all, it's not like you can go wrong, but there's just enough tantalizing in Peterson's role to nudge him to the top for me. But I sure as hell wouldn't argue with anyone who went for either of the other guys (or Lacy).
ETA: I'd also suggest that for whatever reason, there's an aura of "legend-ness" around AP that the other guys don't possess. That's sort of trivial, but I think if all three guys put up 150/2 games for a month straight, Peterson is the guy who has the highest resale value, just based on name and perception.