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All other things being equal, go for the disc - you can sell it later on if you want to or need to, you can play the game on other PS3s, or loan the game to a friend for them to try. The game disc doesn't take up valuable space on your PS3 hard drive, and you don't have to worry about bandwidth or download caps on your internet.
The only two reasons I can think of for buying a game you download over a physical disc would be because the game is hard to find or is more expensive than a disc, and I don't think either is the case right now.