I think both are fine backs, and would rate their chances of being successful pros about equal...
But they are decidely different backs, and Jones fits what the Cowboys need better than Mendenhall does... Mendenhall is similar to Barber, and I think he could handle 300-350 carries a year... Jones is better suited to operating in a RB rotation, sharing the carries with a back like Barber...
Since the Boys are apparently set on keeping Barber around, what we should be looking for is a RB that offers a different dimension... Barber is a pounder, a slasher, the "thunder"... so what the Boys need to pair with him is a guy with the outside speed, a guy who can be dangerous as a receiver, the "lightning"...
Mendenhall is thunder, Jones is lightning...
However, if you want lightning, you have Chris Johnson and Jamaal Charles, one of whom will still likely be on the board when the Boys draft in the second round, while Jones would almost certainly cost the Boys one of their first round picks... I think that Johnson and Charles have the potential to be very good pros, too...