Flip a coin. I don't think either are impactful. If you need a RB take Ballage on the off chance he does something. If you need a WR take Moncrief on the off chance he does something. This doesn't really change the complexion of a team.
I would take Ballage. Moncrief has been in the league for a number of years and really hasn't done anything special. Could he do something in Pittsburgh? Sure. But how many players do you ever really see turn into fantasy starters after 4 or 5 years? Not many.
Ballage is entering year 2. I feel his chances are much greater. Could he flop...sure. But I would let Moncrief be someone else's headache.