I really enjoyed this one:
XML is like a VW Jetta: environmentally sensitive, slow, dependable, and terminally uncool. Lasts a lifetime.
Flash is like a mid-80s Corvette: impressive but environmentally destructive. Oh sure, it gets you the chicks for a while, but ultimately most people sharing the road with you are thinking "what's *he* compensating for?" It only lasts a lifetime by decreasing your lifetime.