Well, I've heard so many bad things about CrossFire. Single card for AMD seems fine, but CrossFire seems 50/50. Some folk like Roadhog don't have any major issues. Other's have problems left and right. I know many of the problems are likely ignorance/user error. Some, however, are legitimate. And I've never used CrossFire myself but have used SLi quite a bit. So I voted for dual 560s.
HOWEVER, I'd imagine the 6970s would offer greater performance. So if you aren't opposed to CrossFire I'd say 6970s. Honestly, if I had the money to spend I'd do 6970s just to give it a go. If they worked well and didn't give me any issues I'd keep the setup. If they did give me issues I'd sell 'em and go 560 SLi.
So I guess in short this isn't much help.
I'd say 6970 CrossFire would offer better performance, and potentially run cooler while consuming less power. But YMMV with CrossFire is what I'd say. SLi seems to be a more solid, refined multi-card solution so you'd probably encounter less issues.
Eh, don't think I helped any.