I think XFX lets you OC under the warranty too.
eVGA has the better warranty, but it's not applicable for the most part for 7900GTXs, you're not going to swap out the heatsink and there are no graphics cards expected in the next 90 days, which means that the step up facility isn't going to happen. As such XFX's warranty is equal.
I don't know about quality in general, but eVGA did have a bad batch of 7900GT cards recently, i can't remember XFX having the same.
I've got an XFX 7800GT, i'm kicking myself for not getting eVGA (as i'd now have a 7900GT(X?)) now, but your situation is different to mine.