Vega if you want to make any use of the full 144hz. I'm running the XR34 on my Fury X and the framerates are very playable (obviously). I'm currently going through Witcher 3 again with a couple of mods (HD textures and lightning primarily) with most settings on High (even Hairworks) and a few on Ultra (some just decimate your framerate like vegetation) and getting no lower than 45 but really no higher than 70-80, which is perfect for my freesync range -- but not really beneficial if you have 144hz cap, that's a whole lot of unused potential

Though Witcher 3 is a sort of 'worst case scenario' as it's hugely demanding but it gives you some idea. Basically, wait for Vega. I'm gonna be upgrading too when they release their offerings -- assuming they're not crap.
Edit: Just to throw in there that 4GB on the Fury has only ever been a limiting factor with very specific settings on GTAV - total hogwash and misinformed conjecture that it's in any way inadequate.