Well I have never done watercooling, but the BBA and Sapphire both use the reference heatsink, they are the exact same card so it won't matter which one you purchase.
I'd go with the Sapphire Radeon... I've read that it doesn't come with a coupon for Half-Life2 (though I got one when I got my 9800 XT), so just make sure you check if that's important to you.