It would never come down to those two. Sheets is a running back and Robinson is player who can play half back, full back, and is a great special teamer. There is no comparison.
Sheets will be a 3rd down specialist at running back and Robinson will do what he's been doing: catching passes out of the backfield, lead blocking sometimes, possibly running the wildcat, and kicking ass on special teams.
Robinson is an under-appreciated backup who does quite a few things well. Sheets is a rookie who does one
thing very well (run the ball). In conclusion, I think we need both on our team.