Speed and memory
In our performance index the Radeon RX Vega 64 scores 17 and the GeForce GTX 980 Ti scores 16, which makes the Radeon RX Vega 64 about 6% faster. That is a small gap, below what most people notice without a frame counter.
The Radeon RX Vega 64 has 8 GB of video memory and the GeForce GTX 980 Ti has 6 GB. That is a small difference and unlikely to decide anything.
In our estimates for popular games at 1080p on High, the biggest gap is in Red Dead Redemption 2 (62 against 58 fps), and the smallest is in Call of Duty (44 against 42 fps). Games that are light enough to run above 200 fps on both cards show little difference at all.