Higher frame rates are better for the eyes in VR. 120 is obviously miles out from being close to fps you are gonna get in VR. So 90hz + 60 fps is the best case scenario. It is unlikely anyone is gonna get much more than 30 or 45 in VR on MSFS (if you can get solid 60fps, lock it there and go 90 or 120hz by all means and I will cry with envy).
Therefore, given the constraints, it is best to lock a steady fps at 30 or 45 fps and use 90hz refresh rate.