ASUS and Gigabyte are probably the top two non-reference GTX 670s so far. I have the Gigabyte, and I'm very happy with the custom cooling. The tri-fans are extremely quiet and the card stays very cool. When the fans do ramp up it's more of a "whoosh" sound rather than something more audible and annoying.
I ended up with a card that has an average overclock, but I'm okay with it. I can manage a stable +125 offset for a 1,249MHz boost clock (my stock boost is 1,124MHz). Some people are able to break 1,300 on the Gigabyte card.
If I had my choice right now and both cards were available, I would probably choose the ASUS, as I'm a fan of their products. I bought mine on release day though and the ASUS still isn't available in the US (rumor has it that it will be released this week though).