I have the Pimax 4K and Vive. A few things of note (YMMV):
1) The Pimax's black levels are considerably "raised" in comparison to the Vive. Gamma etc. dont help this as its a facet of the LCD panel. This affects perception of depth = Vive wins
2) The Pimax has issues with gyro drift. At worst you have to recenter the headset every 10 seconds (when in open play with wing). Other times, it can be every few minutes. Can be a pain when in combat. Vive wins (for now). Pimax 4K needs an external tracker such as NoloVR (not yet released). I've tried using an ED Tracker, but ED doesnt allow headlook mode when in VR (obviously), so this doesn't work (even if you turn the Pimax's gyro off.) For me, that one issue breaks immersion terribly, as I mainly bounty hunt.
3) The Pimax software needs a lot of work. It is quite buggy; still has resolution and format issues, and can require the planets to align before detecting the headset.
4) Comfort. While the Vive is no comfort king, it is easier to wear for longer periods than the Pimax. That issue is mainly caused by the sharp edge to the nose section of the Pimax. Using some foam tape helps somewhat.
4) The Pimax's resolution is clearly superior to the Vive.
5) The Vive is a bit of a annoyance with ED due to the use of fresnel lenses. The joints in the lenses tend to create halo effects to bright areas of the image. An issue that ED developers could help by reducing the brightness of the armchair lighting panels. While the Pimax lenses aren't great (small sweet spot), they dont suffer from the fresnel effects. Pimax wins there.
There are other things, too, but these are the main ones for me. FWIW, I've put the Pimax to one side and likely wont use it again until I have a good solution for tracking.
HTH!