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