I have been offered a choice between these two cards, and I'm not sure which one will perform better in linux at this point. If it was Windows I would just go with the Nvidia and call it a day, but the driver situation on Linux has me leaning towards AMD. Is the AMD the right choice?
Btw, if you ever need to compare GPUs on Linux, use https://openbenchmarking.org/vs/Graphics/
https://openbenchmarking.org/vs/Graphics/AMD+Radeon+RX+64,NVIDIA+GeForce+GTX+980+Ti
The performance between the two -- VRAM aside -- is apparently significantly closer on Windows than in Linux, which I assume why OP is asking.
https://www.videocardbenchmark.net/compare/3218vs3808/GeForce-GTX-980-Ti-vs-Radeon-RX-Vega-64
The AMD card tends to slightly pull ahead on Windows too, but it's a much closer thing there.
In the meta-review at launch of the V64, it was 95-98% of a 1080:
https://www.3dcenter.org/artikel/launch-analyse-amd-radeon-rx-vega/launch-analyse-amd-radeon-rx-vega-seite-2 (German but the table is self-explanatory)
980 ti was about 7% behind a 1070 at its launch; over 20% behind a 1080: https://www.3dcenter.org/artikel/launch-analyse-nvidia-geforce-gtx-1070/launch-analyse-nvidia-geforce-gtx-1070-seite-2
I'm leaning towards declaring PassMark's GPU bench not being a very representative benchmark.
A very safe assumption.