It depends on what you currently have. I would say wait until dx10. However, those are good mid-range cards and maybe that is not your interest.
I would go for the 7900gt beteen the two personally. But performance is very similiar.
Go evga7900gto, its got a nice cooler, 512mb of memory, and evga has the step up program, so if you can't wait for dx10 cards, get this and trade in later.