Get the one that's best for your money. Or if you see one with a good cooling system like MSI's Twin Frozr II, then go for that if you can accommodate the price premium as yiou'll get better over clocking out of them.
Otherwise, even the factory over clocked ones will perform roughly the same with the reference ones, the main thing to look for is price, manufacturer warranty & likelihood to honour warranties and if there is any after market cooling on the card.