You'll be getting approximately the same performance but I would suggest 2 GTX 560ti in SLI, the HD 5970 is loud and noise more hotter than any other card, you'll get it used that means it will be running as a vacuum machine and so hot.
Besides, the HD 5970/6990 failed in some benchmarks dues to noise and heat issues, unfortunately no firmwares or drivers fixed those problems.
Would be great if you're talking about the GTX 560 TI not the standard ones.