Sapphire cards are very solid, I've started buying them since 4870x2 and no one of them has given me issues.
If your sample overclocks to atleast 1200 Mhz it is very worth to get a 7970, otherwise a GTX 680 will be better
Anyway performance is so close that you can decide on your favourite brand, be it AMD or NVIDIA
Maybe take a look at benchmarks and see which brand performs better on the games you are interested?