The short version is it’s pretty much a wash and they’ll look similar, but it depends on what resource of CPU or GPU you’re running out of first. OpenXR is about GPU and MSFS render scale is more about CPU and GPU together.
The MSFS render scale is the sims internal resolution it is aiming for, so if that’s at 100% then it’s effectively the screen or panel’s native resolution. If it’s less than that then it reduces CPU and GPU resources as it’s rendering to a lower resolution, so things like Level of Detail (LOD) and the like are different (less work, less detail to do). It uses antialiasing (TAA) and a sharpener (CAS) to upscale the output if it’s less than 100% (and oversample it if more than 100%). So this then helps the CPU and GPU to say MSFS render scale at 80% because it’s like you are reducing resolution for framerate, but it’s still outputting what’s asked for by upscaling using TAA + CAS. If you set your resolution to 1920x1080 but using render scale of 50% then you get a 1920x1080 drawn to the monitor, it’s just the sim is only doing work for a 960x540 but then scaling up.
OpenXR custom render scale is a mulitplier used on the view sizes ‘advertised’ by the system as what the headset supports, and what motion reprojection would use to synthesize frames. This is really just about the GPU and your VRAM. What made things complicated was that OpenXR on WMR devices would actually reduce the internal resolution if you were low on VRAM, to trade off framerate for resolution. So in some ways it was a bit like the SteamVR’s supersample resolution value set to ‘Auto’ but rather than a metric like FPS it uses a set value of how much VRAM you have on your GPU. On a 2080 with 8GB it would automatically set the resolution to below Reverb native resolution (70%), causing lots of confusion. The way around it was the OpenXR dev ‘Custom Render scale’ values, which allows you to set it manually. 100% should be native resolution on your panels. A 3080 won’t reduce OpenXR custom render scale on auto anyway. If you aren’t using motion reprojection then you don’t save an awful lot making this lower, unless you have a GPU that is running out of VRAM.
So yeah, it’s a bit complicated, but the simplest thing is just to 100/100% and then turn down settings in the sim, like clouds, terrain etc if not happy. Because of the Reverbs high resolution then you can also do things like MSFS 80% / OpenXR 90% and hardly tell the difference, but get slightly nice frames - it’s almost infinitely variable. You can use motion reprojection and 100/75 and it will be fine as well. The trouble is people have very different tolerances for framerate so any hard advice is always arguable.