For pron 8k is the winner hands down, all testers agreed that video is simply better on the 8k, better sde, brightness, colors and blacks, this is a case where sharpness apparently does more harm than good for the 5k, check https://www.youtube.com/watch?v=1lhMRKU5iUk .
In the rest of aspects, it greatly depends on Pimax. If Pimax can get Brainwarp or smooth rendering working on the 8k, it would probably end being the superior HMD since it has lower SDE and all of that can greatly improve clarity with high SS, also if they improve the upscaler rendering and so on. It also depends of your capacity to get the latest graphic card that can feed a higher res image to the 8k.
So it’s a bet, right now the wiser choice is 5k+, it’s cheaper and only loses to 8k in color, blacks and SDE but has much better clarity, but if you’re really thinking long term and trust Pimax will do their work and improve all those aspects, 8k could be a better long term option.
The thing is that on the long term the 8KX could be the superior headset, but we don’t know anything about it yet, even if they are able to produce it, also we can imagine it could compete with StarVR for the professional market and may be ridiculously expensive.