Inb4 The 970 has 3.5gb vram.
But in all honesty the 970 is still a good value. If you are die hard green I would wait until the aftermath of this 3.5gb debacle. There will be quite a bit of used but perfectly usable 970's around the marketplace.
if you aren't against going red. the r9 290x is a pretty good value these days going about the same price as a 970. Some even far below its asking price.
Here is a little graph to help you
It's almost double the performance of a 5870.
It won't crossfire with a 5870 so You could contact xfx to see if you can send in your other one for a 270x but I doubt they will give it to you unless there is something wrong with it.
You can't compare them like that due to them having very different architecture. Nor can you directly compare Nvidia to AMD specs wise. Its kinda like comparing Intel to AMD saying why wouldn't this win because its clocked faster.
Maxwell is just more efficient at using its memory due to its very unique architecture.