Here is a tough one. Vickers improved a lot and has shown he is a very good lead blocker and an all around FB. McClain was a starter in '08 and moved to lead blocker in '09. I don't think there is a wrong choice, but I am not sure on who I'd chose right now.
Vickers doesn't give much as a ball carrier/receiver unfortunately, but he is like having a guard in the backfield as a blocker and being that this is the AFC North team, we'll have great backs so I'd rather have the superior blocker in my fullback