I think its a matter of what you want your RB to do. I will agree that Gurley is a better runner than Elliott, that said, Elliott is a better pass blocker and pass catcher. So, if you value RBs who are strong in the passing game, you could argue that Elliott is a better prospect because of superior versatility. The same way a person could make an argument that Jamaal Charles was a better RB for the Chiefs than Adrian Peterson would have been. Peterson is a better, more powerful runner, but Charles value in the passing game is why the Chiefs would likely have not made that trade.
I think Elliott is a better fit for what Dallas wants, than Gurley would have been.