Well your stuck between a rock and a hard place my friend. Both of your graphics cards are pretty awsome but overall it comes down to what your computer can handle such as: can it support wattage requirments and/or sapce on the motherboard or maybe can it actually fit in your case and if you'll notice they both take up one slot but are so WIDE that they will probibly cover up to slots (depends on which slot you use and/or your mother board) and there is a MASSIVE prise diference as the 690 is $1,000 and over and the 680 is $500 and up. I've been looking into graphics cards for a while and i've just boaght an ASUS HD directuII TOP and it is a absolute machine and it was only $390.00.
Just remember there requirments do a bit of reserch and if price is really not a problem and your computter can handle it i would defiently go for the 690 defiently because it houses two 680! so your basicaly buying the two plus an extra!
hope it helps!!!