I would say go for the R9-280X. The 3GB VRAM might come in handy at times.
Examples of the situation where the extra VRAM might come handy are :
1. Playing BF4 for long durations. So even if the RAM utilization goes above 2GB, still there would be no frame drops with the 280x, wheras if RAM utilization ever goes above 2GB in Nvidia 770, then you are going to see frame drops.
2. Playing Skyrim with high resolution texture mods applies. I mean anyone who plays skyrim mods it with high def textures and all which causes an increase in VRAM utilization. And 280x has 3GB VRAM which really comes in handy.
3. Playing GTA 5, I mean you need some VRAM to load all the textures and other stuff.
So, looking for the future the 280x would be better option.
Though if you can go for the 4GB 770, then that would be awesome. But I am not quite sure that the 256 bit bus would be able to utilize all the 4GB.
The 280x has 384 bit wide bus so, it can utilize more than 2GB RAM without a problem.