I can't offer any empirical performance measurements, but I had an absolutel nightmare of a time with the drivers for the last (and only!) pair of ATI cards I used, and their support left me totally cold. I ended up using only one card and then eventually just threw them both away (close to €1000 worth of cards at the time) and bought nVidia instead.

I won't touch ATI with a barge-pole ever again, but that's entirely down to my personal experience with them.