Speed and memory
In our performance index the Radeon R9 280X scores 5.5 and the GeForce GTX 960 scores 5, which makes the Radeon R9 280X about 10% faster. That is a small gap, below what most people notice without a frame counter.
The Radeon R9 280X has 3 GB of video memory and the GeForce GTX 960 has 2 GB. That difference matters at 1440p and in new games at High textures, and it can outweigh a small speed advantage as the card ages.
In our estimates for popular games at 1080p on High, the biggest gap is in Red Dead Redemption 2 (16 against 11 fps), and the smallest is in ELDEN RING (14 against 13 fps). Games that are light enough to run above 200 fps on both cards show little difference at all.