Fair enough. If they can't really be compared then because of their different roles, then how do we know who is better?
Admittedly, I don't watch a lot of either. But when I watch Real, I find Di Maria a very frustrating and wasteful player. When I watch Bale, you can say he is to Tottenham what Ronaldo is to Real (to a lesser extent of course).
I think Bale would be perfect for Man Utd., but I wonder if his style would work outside of the EPL.