I've tried a bunch of different 7950s and the Sapphires are, by far, the best. I know this thread is about 7970s, but maybe this'll help someone. anyway. All numbers here are mining BTC but I've gotten similar KH/s doing scrypt.
My favorite is my Sapphire Dual-X that's doing 633GH/s with cgminer at 74C with the fan under 50%: engine=1200, mem=1050, vddc=1.169, I=7. It ran at 1210/1060 for 637GH/s for about a week but the driver crashed needing a hard reboot so I dropped it back a bit.
I have a Vapor-X which'll do 620MH/s (1180/1030) but crashes when I push it any faster. The MSI Double Frozr tops out at 610MH/s and the XFX single-fan card crashes any higher than 595MH/s. Worst of all, though, the XFX ran hot and loud because the cooling system was crap. I'm not sure how much of a difference the Vapor-X makes but it does add about 1/4" to the thickness making it hard to fit into some cases.
Speaking of which don't forget how important the case is. My main rig is a Coolermaster HAF that runs the cards a good 20C cooler than a plain, generic case even on its side with the side off. If I lived in a house with a basement I'd do crate rigs with box fans but since I live in a small apartment noise and heat are key.
They're just about the same. Linux draws somewhat fewer watts (CPU is less busy) and it needs less RAM but Windows usually recovers from driver crashes more gracefully. One of my Linux boxes can be rebooted on the command-line if the driver hangs but the other needs a hardware reboot, which sucks if I'm not home. My Windows miner can almost aways be reset by bouncing cgminer.