The 2 cards I believe perform very similar except the 280x uses more watts and produces more heat but can use all 3gb vram. The 960 4gb is a bit of a gimmick as it can barely use more than 2gb vram due to it limited memory bandwidth. If gaming under 1080p I would pick the 960, if 1080p I might be tempted by the 280X but I'm not a fan of AMD gpu drivers