I agree with CBZ. It is an excellent question .. and a very intriguing one as well. It's a hard question to answer if you don't have a biased viewpoint.
I admit that my bias leans towards Barber for several reasons. First, though, I'll give Jones credit where credit is due. JJ can break one at any time. He's got the long run potential. Often times, I get frustrated watching him blast straight into a callapsing hole though. It's like he has great vision on some plays and not so great vision on others.
He seems to produce well on draw plays (which is why I think Parcells is "starting" him), but on other plays he really isn't all that impressive. He can't catch out of the backfield either .. and overall, he's just inconsistent, IMO.
Marion Barber, in my opinion, hasn't had the chance to show what he can really do. I like the fact that he can, if need be, run into a callapsing hole and push some players backwards. He's deceptively quick; able to break tackles; able to gain yardage after initial contact and is a great back to put in with fresh legs against a tiring defense. He's quick; and punishing at the same time ... and if I had to pick choose one of the two backs to hang my hat on for the next 3-4 years, I'd pick Barber.
But like we were saying, it's an interesting question ...
I believe the correct answer is just be glad that we have both players and that both of them are relatively young.
With that being said, keep an eye out for Barber in the up and coming weeks.. He may be the next back to rush for 100 yards ...
..and having two quality backs that can get you 100 yards in the NFL is ...
Priceless.