At the risk of sounding like a homer, I actually think Barber and Jones are each better than Betts. Barber and Jones are not that far apart, and Betts is not on their level IMO.
Granted, I haven't seen each of Betts' carries, but his numbers aren't all that impressive. A lot of times, backup running backs are said to get a higher yards per carry (see: Troy Hambrick, LaMont Jordan) due to teams not game planning for them. Yet Betts' average and overall statistics are very average.
With that said, I think the gap between Portis and Jones is much greater than being given credit here. Portis is a top four back. Jones looks like a top-four back a couple of games each year. There's a big difference between those two talents, until Jones proves otherwise IMO.
As far as depth, yeah, I'd agree the Cowboys are 'less screwed' if Jones goes down than if Portis goes down in Washington. But like Biz said, that's because Portis is such an immense talent and cannot be replaced.
It's like saying Philly has better depth because they'd be less screwed if Reggie Brown got hurt than if we lost Terrell Owens.
Portis, alone, carries Washington's running back rotation. Betts is a solid complementary player, but nothing special. Meanwhile, we have two slightly above average running backs who are younger with that dreaded "potential." They look great some weeks and pedestrian others.
I'd take our backup but their starter. Let's put it that way.