I believe this is a situation where both sides are correct. JJ is a better rb than MBIII, but the line is not blocking well...therefore, we're better served with MBIII, the more powerful runner and the better passing down guy.
When the line is blocking a little better, JJ will pop through from time to time and gash the defense for more yards...he'll make defenders miss and he'll scoot for long runs and TDs...we've seen it.
However, MBIII is clearly a better blocker and receiver right now in the passing game, and MBIII is a more powerful runner, which allows him to push the pile, make the better runs in short yardage situations, and avoid the stops in the backfield. When the OL is struggling, I think MBIII will do slightly better than JJ, but that little bit is important.
When the OL improves its run-blocking, JJ will be the RB with the most potential and franchise star power, imo.