Completely different architectures. It all depends on the throughput of each component. Comparing the these two cars is like comparing a Viper to a Lamborghini.
Anyway, I would advise the HD 7970 for one simple reason, VRAM. You very wisely got the 2GB version of the HD 5870 three years ago, and you want to make the wise decision with this card too. Also the Radeon can be overclocked by 20% or so, and it has 50% more memory bandwidth. I think it will definitely last you longer than the GTX 670.