Listing your specs would be helpful, are you upgrading or just new build?
It depends with these 2. They are pretty much on par against each other. It also depends the prices on these two. Due to bitcoin mining, amd cards went up on prices. The 770 here costs less then 400 in Canada, while the 280x costs a overpriced 600$!
It also depends on the games, a lot of games will favor the nvidia side, and some games will favor cards from th AMD side. One advantage that the 280x has is a extra GB of vram. = better for higher resolutions. You'll have to sli the 770 for better gaming with a 1440p monitor. So the AMD side could be a option to take in idea.
The gtx 770 is a great choice, and same time the 280x is a great choice as well. and when the g-sync monitors come out, you should get a g-sync monitor if you pick nvidia.