you should know that the sapphire tri-x 290 is an excellent card and out of the box is slightly faster, although barely noticeable, than most regular 780 models like the evga acx 780. you should not overlook it. but all things considered, the 780 will overclock farther because the gk110 in the nvidia card runs cooler than the hawaii chip in the amd option. sapphire is a very reliable company with very good support and warranty. if you wont be overclocking the r9-290 is a better option, if you will be overclocking, the 780 is probably the safer bet.
im not sure what model you are looking at for the evga card, but the superclocked acx cooler version is evga's best air cooled bang for buck in their model list.