uhmhm $200 vs $500 uhmhm
a single 560 ti will cost you 200$ bro. not two 560 ti's So... 400$ 1GB's of vram (Because vram doesn't pool in SLI), More heat and power usage, and microstutter with Scaling.
GTX 670: Power of a GTX 680 without spending a extra 120$. Less power requirement, better performance. No micro stuttering. and access to the latest technologies like GPU Boost and FX AA and many more!
GTX 680: same GK 104 as GTX 670 just a little more standard powerful.. Idk bro you do the math. 1GB of vram vs 2GB of vram....