ASUS without a doubt!
ASUS cards have so much going for them.
Check out this feature set: http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/55270-asus-gtx-670-directcu-ii-top-review-2.html
I don't know anyone who places this much quality in to their product. The propitiatory manufacturing processes ASUS has are really unique.
Definitely go for the ASUS Direct CU TOP
edition as it is faster than the reference GTX680, $100 cheaper, and it is also silent. This benchmark proves it: http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/55270-asus-gtx-670-directcu-ii-top-review-5.html
Also check out JJ's video review of the card. He gives a better overview of what makes this card so special.http://www.youtube.com/watch?v=2Y2GoT-hRb8&feature=related