Definitely Quick Cut.
As a powerback he will mostly be running inside so he'll slow down as he runs through the line. In a discussion about First Step Bort has previously said that Quick Cut is best for keeping up your speed through the line.
Originally posted by Bort
The RB is usually up to speed when he hits the hole, so first step wouldn't even have a chance to do anything. Quick cut is more useful, I'd say, for moving quickly through the line.
http://goallineblitz.com/game/forum_thread.pl?thread_id=1472154&page=2