Sapphire has better overall quality, but has a worse warranty than the XFX. This is from my own experiances. The XFX 7970 if NOT a GHZ edition will be pretty close to the boost edition Sapphire 7950 performane wise. The XFX still being faster overall though.
I would go for the XFX 7970 over the Sapphire 7950 but make sure to register the card for warranty purposes just in case you need to use the RMA process. You should be able to overclock the 7970 to the ghz edition 7970s clock speeds pretty easily as well to gain a little extra value.