Which is the better Graphics Card?

Solution
The GTX 960 lands in between the 280 and 280x, so the 960 is the better of those 2, especially when you consider how much you can overclock the 960. It'll also run a lot cooler and more efficiently.
The GTX 960 is rated at 120W while the R9 280 is rated at 250W, so that is an advantage.
The GTX 960 is also a little faster.
The R9 280 however has 3GB of VRAM compared to 2GB, which is an advantage in games with high memory usage like Skyrim with mods.

If your power supply can handle it, I would probably choose the R9 280.
If you can afford the extra money, I would choose a GTX 970 over both these cards.
The GTX 970 is rated at 145W, has 4GB of VRAM (best to keep usage under 3.5GB) and is substantially faster than either of these cards.