Go for the 680.
With SLI you never actually get twice the performance and it just opens up a whole world of problems. There's twice the risk (two cards that could go faulty instead of one), you destroy the airflow and raise temperatures within the case and not to mention the endless hours you could spend diagnosing graphical faults just to find out it's because there's always issues when you try and run 2 cards together.
In my opinion (not always right, I know
) go for the 680. It's an amazing card and will do you fantastically for at least 4-5 years. I still use a 5 year old card in my spare gaming rig and I can happily play BF3 on it.
Hope this helps