The 285 is presumably faster than the 280 (not tested in the chart below), so the 280 and GTX 760 are really right about the same performance level. If performance is the same, you need to look at some other factors.
The arguments for the GTX 760 would be the fact that it offers a lot more unique graphics settings (TXAA, FXAA, HBAO+, DSR, etc.), it will allow you to enable PhysX, it has Adaptive VSync, there are quite a few more games that are optimized for Nvidia hardware, and it comes with Nvidia driver support. If none of those things matter to you, then the 280 might be a consideration. For me personally, PhysX, Adaptive VSync, and DSR are settings I wouldn't want to go without if given the choice...