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?
It's not even a contest between the two. The 980 ti is at best 1070-class while the V64 is closer to the 1080 than the 1070ti.
If we extrapolate from Kepler EOL, Nvidia will probably drop Maxwell support in the next year or two. Given there will likely never be a free driver for Maxwell, you'd be stuck using sucky old proprietary drivers in the not too distant future.
With the Vega 64, you've got world-class kernel and userspace driver support.
The biggest downside of the V64 would be that it's quite power hungry but the 980 ti isn't much better and if you limited the V64 power draw to the level of the 980 ti, I'm sure it'd still be faster.
Why in the world would you want the 980 ti, especially for Linux?
I was using a water cooled VEGA 64 up until last year. I could play Cyberpunk at 3440x1440 med/high settings and pull around 40-50 fps. On a free sync monitor that is plenty for a smooth gameplay experience. But as far as I understand Linux doesn't support freeSync yet which is a shame.
If you can water-cool the VEGA 64, there is a lot of performance that is still left on the table. You don't even need to overclock as the drivers will push for as long as you have power budget and thermal room.