I'll discuss two models:
The ASUS model is a 3-slot card so it's large. However that also helps keep it cool and quiet. It's clocked at 800MHz (lower than some) which makes it even quieter though you can easily overclock and still be quieter than other cards.
The Sapphire is 850MHz and probably quiet/cool. (too many different models to keep track of). It's a 2-slot card which is typical.
I had an awesome Saphire HD5870 Vapor-X (still running) and I love my new 3-slot Asus GTX680 (similar DC2 cooling setup).
There's also a cost difference.
*You have to be very careful about benchmarks. Some pick and choose the games so a particular series looks better.
You also need to factor in the GAME BUNDLE cost. (Is there one for the Sapphire or not? I thought it was for all HD7950's...)
I'd recommend the Asus first, and if you consider the Sapphire which should be also a great card be sure to verify the gaming bundle is included as I see no info on it here.