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.