hahaha, I love hypothetical matchups like these.

I guess it depends who wrote out the fight, Tolkien or Rowling. Obviously the character belonging to whoever authored the fight would win. If we're basing the outcome on the merits of the characters alone I'd have to go with Gandalf because (and I'm in the middle of the "Order of the Phoenix" right now) Dumbledore is always being alluded to as an extremely powerful wizard, yet he's never really done any "fighting".

Back when I was like 13-15 I read the crap out of this series of books called Dragonlance. There was a character who was a mage in these books called Raistlin, one of my favorite and most dynamic characters I've ever encountered. I believe that he'd mop the floor with both of these guys.