SLI - GTX 970 different brands?

Yeah, you'll have zero problem. You can in fact even SLI/Crossfire cards from different generations in case the chip itself is the same, i.e. 290x/390x come the next update from AMD. Unless I'm mistaken it's exactly the same story with Nvidia. In any case the brand doesn't matter for a same model SLI as Nvidia is the one that provides the GPUs to all manufacturers so they're the same across the board with minor difference in clock speed. However, like Rogue Leader pointed out - the SLI configuration will use the clock speed of the GPU/VRAM of the "weaker" card. Not that it would matter with so much power :))).
 


You are mistaken, SLi does not work that way.