If you don't care about the price premium of the 5970 then the 5970. you'll get micro stutter from any sli crossfire set up. Micro stutter is only really noticeable at lower frame rates which will truly be a rare occurrence with the 5970.
The gtx has quite a bit more on transistor count one because, of it's bigger cache, and I believe because of it's more robust cores. Since they support a number of programming languages, plus physx and cuda technologies. But performance wise the radeons have better transistor to frame rate if that's what your getting at