Im not too sure about the 7970's but If you are looking to save big bucks then I would check out the XFX 7950 DD on dabs for £310 which is a brilliant price and with 3 games too, it overclocks like a beast and beats a reference 7970 with no problems, clock for clock the 7950 is 1-4% slower than the 7970 and the 7950 can achieve higher OC's than a reference 7970 in some cases for example the HIS ICE-Q 7950 beats an XFX refernece 7970. But that is in my opinion.
As for the 7970 I would go with the sapphire, the MSI is IMO overpriced and not OC'd that far.