i think the 7950 is better than the ti, and amd has better multi monitor with eyeinfinity, but i know for sure in certain games that they prefer nvidia, either is a good choice, at this time i dont think sli or crossfire is worse than the other its more on how stable the drivers are. Im crossfiring two 6770, but there lower end cards and dont scale well as i couldnt bump settings in some games because of the 1gb vram.
SLI 660s if i remember beat a single 680 and of course two 7950 would beat a single 7970 lol