That is a little bit of a tough call. Resolution wise, they are pretty close, but P4Xi has better contrast. Aside from the objective size, that is the thing that helps P4Xi in low light.
Both have good turrets. I think P4Xi click feel is a little more firm. I do like the covered windage turret of the MPO though.
Depth of field is a little better on the P4Xi, but MPO give you a little more flexibility with its greater magnification range and it is smaller.
They have very different reticles and MPO's N-OMR reticle has really grown on me, to be honest, and I like tree reticles anyway. That having been said, SCR reticle in the P4Xi is very good and has been around for quite some time. We know it works because there are thousand of people using it out there with good success. If you are primarily a dialer, SCR is a very nice design. N-OMR does have better visbility on low power.
P4Xi is a bigger scope. For a bolt gun, I'd probably lean toward the P4Xi. For an accurate semi-auto, I think MPO is a more flexible option and, honestly, I use the reticle for hold-offs more on semi-auto guns than on bolt guns.
ILya