Here's a good video which supports both of our arguements:
https://www.youtube.com/watch?v=pEWABNJi6Rc
You can see that in games like Dying Light, DayZ and Titanfall, which have a lot particles or graphical information, the 1Gb extra VRAM and higher bus width makes the R9 280X better in those situations.
But in other situations the GTX 960 beats it.
It highly depends of game and game area.
Also, in many games, surely the VRAM amount plays a part.
Games like Dying light easily uses over 2Gb. Thus 4Gb 960 would surely do better in them.