The Julius Jones/Marion Barber debate is the subject of this article. The problem is the author starts with a statistical supposition.
Comparisons of statistics over a one-year period is often misleading, but even conceding that last year they were interchangeable, there's not much debate that Julius Jones is the better pure runner. Anytime Julius touches the ball he has the potential to go all the way. He's a more elusive back who has excellent open-field moves that can lead to big runs.
On the other hand, MB3 is probably the more well-rounded back. He does all the other things well like block and pass catch, plus he is a better between-the-tackles runner when you need a tough yard or two.
So the question is will we have RB-by-committee this year? I would say no, under one condition, Julius stays healthy. If JJ can do that, then he should be the featured back for the running offense, he brings a dimension Barber doesn't possess. But I wouldn't hesitate to use Barber on 3rd downs, short yardage situations including goal-line and for pass-protection.
Overall, it's a great problem to have.