ASUS.
You can read the links, but Techpowerup gave the Asus version a 9.0/10 and the EDITOR's CHOICE whereas the Gigabyte version 8.3/10. In particular the Gigabyte card was "very noisy during gaming" which would be my dealbreaker right there.
http://www.techpowerup.com/reviews/ASUS/R9_280X_Direct_Cu_II_TOP/31.html
and
http://www.techpowerup.com/reviews/Gigabyte/R9_280X_OC/31.html
*Make sure you are comparing prices and reviews for the correct card. The ASUS above is the overclocked or "TOP" version whereas there's a cheaper one with lower performance that I wouldn't bother with.
Also, you can adjust the fan profile likely if you desire to make fan noise slightly less which is really it's only issue.
The only other card I'd bother investigating is the Sapphire Tri-X OC version though I have a feeling it's going to scale in price in order of Gigabyte, Asus, Sapphire (the specific cards I mentioned): http://www.eteknix.com/sapphire-r9-280x-vapor-x-tri-x-oc-3gb-graphics-card-review/20/
I don't recommend the Gigabyte. I'd probably the Sapphire Tri-X OC version if price wasn't an issue, otherwise I recommend the Asus if cheaper.