780 is stronger. The r9 290 is the one that compares to the gtx 780. I would look into the new 970 if you can afford it, if not the 780 is still a great card.
The GTX 780 is much faster than the R9 280X.
The new GTX 970 has similar performance to the GTX 780 but with lower power consumption and more VRAM.
I would choose the GTX 970.