X vs. 3 should be a question of if you need one vehicle size or form factor over another. You will never like a model 3 if you cannot haul people or things you need. You may be satisfied with an X if you get the P100D, but it will never be as fast as a P3D.
Isn't the 0-60 time on the M3 PD 3.5 seconds as compared with 2.9 seconds on the MX P100D? Not saying that the M3 wouldn't be a blast to drive, but I do think the MX is still the quicker car (for something in the range of double the price).
I agree that these are vastly different cars. That's really the most important factor here - I have the MX specifically to be able to haul lots of people/gear up the mountains to ski, or to a backpacking trip, or for occasionally transporting large objects (already helped someone move furniture, and fit a portable air conditioner in without needing to tip in on its side). I came from an Outback, which I loved for this flexibility, so that made a lot of sense for me. If I wanted a fun daily commuter that had great range for road trips, but didn't need as much space, I'd have ended up with the M3, no question.