This is a little hard, as both of those GPUs are great choices. You can see from our review on the Sapphire Radeon HD 7950 Vapor-X OC with Boost keeps up with the GeForce GTX 670 - but it doesn't beat it.
The Radeon HD 7970 is that card, that will come close to the GTX 670 and beat it. But the Sapphire offering does come with 3GB of RAM which is good, so if you're gaming at a high resolution or enjoy throwing on lots of anti-aliasing, this will be a good boost.
Both cards are great - but the Sapphire Radeon HD 7950 Vapor-X OC with Boost does come short of out-performing the GTX 670, but not by much. Two of them in CrossFire X mode is still going to kick some serious ass!!!