HD7990 vs GTX780Ti:
http://www.techpowerup.com/reviews/ASUS/GTX_780_Ti_Direct_Cu_II_OC/24.html
At first glance, an overclocked GTX780Ti gets almost exactly the same performance as a stock HD7990, however remember multi-GPU setups tend to have issues including stutter, RUNT frames, and NOISE. Despite the price difference I just can't recommend the HD7990.
I like ASUS' cooling solutions and quality in general. There MAY be a better version of the GTX780Ti or a similar one for less money and you can investigate that further (some of the EVGA models are good too).
I would start with the Asus GTX780Ti DCUII card and only get something different if you're certain it's better. RELIABILITY and NOISE should be your prime concern as all the overclocked models have fairly similar performance.
(It may seem a minor thing, but I prefer cards with a metal BACKPLATE like this Asus card has. I had a card WARP and that was apparently why it died.)
This Asus card also has red and gold STICKERS so you can choose the color (or leave black).