If you have the money, go for the 690. If you don't care and you just want gaming, get the 680. Avoid avoid SLI all-together. In today's modern technology ... you want to preserve your computer's life. I'm learning now and I know for the fact that my next computer build will not be towards SLI but towards one beasty card. Your CPU, SSD (not HDD or those half-assed hybird drives) RAM, motherboard's specs and that single beasty card will be the key feature to give you what you're looking for. Also, you save on power, less heat in your computer, longer life-span and less pressure/stress on the motherboard. This is my conclusion.
690 is not only better than the 680, less noise, heat and power consumption and should last longer than the 680 itself (looking at the default design by nVidia). It does cost a lot thou'.