I've read all the posts above and have to agree that Gandaf is the more powerful wizard, BUT the poll is not who is the more powerful, but who would win in a fight. And yes there is a difference. Gandaf is a wizard in the Norse God style, way more powerful than Dumbledore, however the magic available to Dumbledore is better for a straight out fight.
Dumbledore is able to appriate (sp), Gandaf can't, Dumbledore can become invisible, Gandaf can't and finally Dumbledore is able to quickly cast a spell, in the LoTR's books Gandaf can't cast a spell anywhere near as quickly as Dumbledore can.
Imagine what Fredos trek would be like if he had access to Dumbledores magic instead of Gandafs. Side by Side appriation to Mt Doom, a quick invisiblely spell and a nice stroll up the mountain :-)