I have both, and any time a game is available on box systems, I tend to go with the PS3 version. Mostly, it's just a comfort thing.
The PS3 is definitely better value. As mentioned above, blu-ray + free wifi + better reliability makes the PS3 a good choice.
Both systems have great exclusive games. Halo, Mass Effect, and Gears are all cool, but so are Metal Gear, Resistance, Killzone and Uncharted (with God of War on the way).