Crossfire scaling is fine just depends on the game. CF 5870s should crush all games... I should know since my OCed 5850s do great. Crysis and Metro 2033 at highest settings 1080p DX11.
As for 3D, I'd say you're right that Nvidia has the better solution. I only know a little about AMD's 3D stuff... seems like it should work with the 3rd party software running but I can see it might have issues.
I'm pretty sure the 560 is basically around the same performance as a 5870. The only thing you'd gain is better 3D support.
You could maybe buy a smaller and significantly cheaper TV lol... or even just a 3D monitor... Personally I find higher pixel density to be pretty important - but to each his own.