Personally I think we got real lucky with Karlos and Mike, and that RBs are not a dime a dozen. How bad were Boom Boom, Boobie, Reggie, etc while Shady, Karlos and Mike excelled? If we have the room, I say we keep Mike on, because running is vital to our offense. If I had more faith in our passing attack, I'd be more likely to move on from him and take the 5th round pick.
Edit to add: I wasn't impressed with JWill last year, but maybe that was just being a rookie.