GTX 960 or R9 280x?

The 280x is slightly faster in pretty much every game and resolution. It doesn't really matter if it is an Nvidia "optimized" game either like Far Cry 4. Add to that the price difference, I've seen the 280x going for well under $200 now with rebates now that the R9 300 cards have come out. So, as long as your PSU is powerful enough, you should get the 280x no doubt. 30 amps and 550 watts is recommended but you can probably get away with 10% under that with a good quality unit.

$170 XFX 280x on newegg after rebate + free game: http://www.newegg.com/Product/Product.aspx?Item=N82E16814150706&cm_re=280x-_-14-150-706-_-Product
 
Far Cry 4 is very well known favoring AMD card. It is one game that even AMD use in their own slides showing Fury X is fasster than 980ti. But other games might not like that. In hardocp test 960 and 285 (which also behind 280X) able to perform significantly better than 280X.

@OP

In general 280X is the better deal unless power supply is an issue to you