I'd probably take Megatron, I think the gap between him and the second best receiver is probably even larger than the gap between A.D. and the second best running back. Also, even the best running backs usually slow down a lot during their early 30's, I think Megatron might be able to have great longevity.
The only teams that should chose Calvin over AP are the Ravens, Seahawks, maybe 49ers and Texans, only cause they already have stud running backs.
The Texans, in this hypothetical, are the only team where you could either give them one of the scariest WR duos or scariest RB duos of all-time. Either way they went would be incredibly scary for opposing defenses.