personally i would normally say single card, but i Think 2 670s is more worth it in this situation if you avoid possible heat issues only because of the fact that if ever necessary, its much easier to sell a 670 than a 690.
Well it depends i would get the 690. Nvidia really went all out to make it the best. x2 GTX 670 in SLI no doubt will start to get loud with reference coolers and an overclock. x2 GTX 680 in SLI is expensive and not likely to be worth it over one of the other two cheaper options.
Now if you could find a pair of non-reference GTX 670's for SLI, that would be different and my recommendation otherwise just get a 690.
Get the 690, cards in sli always mean higher temps, lesser games compatibility and of course 2 PCI slots. The 690 is also slight more powerful than a dual 670 and bit, bit less powerful than a 680 sli, besides the look is awesome and in the future maybe you can sli 2 690s and be ready for at least 4 or more years from now.