I run AMD(my laptops nvidia) I've recommended Nvidia to my friend who has a similarish build level as mines back in january. I personally prefer AMD but I dont let it get ahead of me on Price to performance on specified tasks.
preferably, Single card is a much better solution, but when you decide to go 3x 1080p, single card isnt much of an option and its recommended to go dual cards. In a dual card situation, crossfire has a slightly better scaling than sli does. In accordance to frametimes, the 290/290x utilize a new method of crossfire which eliminates stuttering on multi gpu setups, and SLI for kepler based gpus have been consistent so far, so there isnt a problem. Because the 290/290x has no retail aftermarket cooling, OC headroom...