Football Outsiders has a breakdown of MB3 vs. JJ in a new article. They evaluated both players in the Atlanta game, here's a snippet of what they say.
Jones' first run came on first-and-10 on the Cowboys' first possession, and it showed his inability to find openings in the opposing defense. Jones had fullback Lousaka Polite in front of him in the backfield, and tight end Anthony Fasano went in motion from left to right before the snap. If Jones had cut to the outside, around Fasano's block, he appeared to have enough space to pick up at least a few yards. Instead he stayed to the inside, running directly into Atlanta defensive tackle Jonathan Babineaux. He was stopped for no gain.
Contrast that with Barber's first run, on first-and-10 from the Atlanta 21. It was typical Barber: He took the handoff, took a hard step toward the left tackle and then changed direction quickly behind the line of scrimmage when he saw a hole between the left guard and center. He exploded through that hole, and if a collision with the umpire hadn't slowed him down, he probably would have scored. Instead, he gained 13 yards.
I pretty much agree with a lot of their findings, except I'm not advocating that Dallas changes the way they use both backs; the Outsiders seemed to be behind MB3 getting most of the carries. I think Parcells is doing a good job of getting both of them touches in the right situations.
It's a good article, but it's nothing new. I told you the same things in my film review, especially in this paragraph.
We both came to the same conclusions: JJ has better top-end speed but is not being creative in his cuts and is running into the traffic instead of around it. MB3, on the other hand, is good at the initial read of the defense and finding the hole, then quickly accelerating through the hole, and capping off the runs with tough-running. JJ needs to show this week that he can be the elusive, open-field back that we know he can be, or MB3 is going to start taking more of his carries.