GTX 760 SLI will have better performance, but you may have microstuttering in some games, it will use more power, in turn generating more heat, and the on-board GDDR5 RAM will not double, it is shared between them to create images. As for pros, it is theoretically double speeds, although never more than 80% at most, and more performance than a single card priced the same.
You should get just a GTX 770. It will still rip through all modern day games at ultra settings, with some AA.
Two 760's is a kick-butt rig!
But one 770 has its advantages. Less power, less heat (so less noise).
Less fiddling around with SLI game profiles.
Less PSU requirement - with the two 760's you would need 2 x 2 vidcard power connectors and probably 650W minimum.
One 760 would be good-to-go with maybe 500 W (good-quality though).
Bottom line: I would be happy with a single GTX 760! I'm going to get one, too.
But a 770 should be killer. As long as you're not going super high-res, ie. over 1920 x 1080, you won't need more.