Asiata is the short answer.
12thBob makes a good point, if you have a barren bench you might want to consider both. Everyone thought Forsett would get most of the carries against the Steelers, but it was actually Pierce who ended up with the lion's share. Just because Asiata is the guy we "think" will get more carries, doesn't mean he will. It's not a bad idea to grab both to be safe. If your league is somewhat lax on waiver activity, you can probably get away with just Asiata and then grab McKinnon next week if he ends up seeing the ball more.
It will probably be a split carry situation, though Asiata has played well in limited looks last season, and he knows the offense much better than his rookie competitor who's basically been with the team for a month.