just want to sub this thread, i was wondering same thing
main difference... P67 chipset (which Asus has) does not have onboard video (does support OC'ing cpu) Z68 chipset in the Gigabyte has onboard video and also supports OC'n
Since you are using a dedicated GPU, either will be fine. I'd personally go with Asus... I like the software and tools that comes with their motherboards... and I havent had a single issue with either of my ASUS mobos (one is 3yrs old, other is 1.5yrs)
I personally enjoy the port layout on the Gigabyte boards because I use a lot of expansion cards.
The newer Gigabyte boards allow me to use 2x PCI-E x1 devices, a PCI card, and a dual-slot GPU. If I tossed in another dual slot GPU, I will still be able to fit all of that (I believe; haven't seen the layout in a while).