Since I just upgraded to 1440p I was thinking about either getting another 7870 to run in crossfire but in the past I've known there to be huge problems with compatibility in certain games and wondered if that has become less of a problem or should I just get a 7970 instead?
7970
7970. I think people should avoid crossfire and SLI with middle-tier cards. Sure, you get a higher performance value, but you miss out on faster memory et cetera.
The 7970 has 3GB of Vram, that will be very useful on your higher resolution monitor. It'll be much better if you crossfire it with another 7970, because Vram doesn't stack. 3GB + 3GB = 3GB essentially. And I always recommend 3GB as the sweet spot for 1440p