If it was my dollar, I'd go Gigabyte. The X58A-UD3R seems to have more features for the money. I used to run all ASUS boards, but got my first Gigabyte (LGA775) board a few weeks back, and I love it.
I read a while back that if you run two GPU's in the Gigabyte board though, they run at 8x. I could be wrong about that though, maybe someone can correct me if I am.