Dual 680s.
More power, 4GB VRAM per GPU, less heat, air directed out the back, if one GPU dies replace it while still having a graphics card.
Only real con here would be slightly more power consumption and if you consider it a con using another slot.
690:
Slightly cheaper than 2 cards, single card, e-peen, slightly less power consumption, 2 cables rather than 4, no SLI bridge required (built in).
Cons would be one fan for 2 GPUs, dumps air into the case as well as out of it, hotter due to 2 GPUs being fed the same amount of air, 2GB VRAM per GPU, if VRM or a GPU dies whole card gets RMAd.
A 690 will hardly be cheaper 2 years from now even when they are technically obsolete. Quad-SLI also has no real effect on games as the drivers really aren't that optimized for 4 GPU load balancing. With the way things are looking, next years 780 will be about 25% more powerful than the current 680 so I'm guessing for the amount of money a 690 will cost next year you could get 2 780s lol.
