Ride them both and see what you like! Personally I think you would be splitting hairs and it is rider preference.
This^
I have a '12 Pro, coming off a M1000 mod.
I bought it because it was a screamin deal last year, not because I HAD to have a Pro. Love the chassis. As easy to maneuver as my M7, but more user friendly. In stock form I think it will lack hp more than the other 800s at altitude. My riding is all 1000-4000' now, so it has good power.
Got the chance to ride a XM last weekend.
The ETEC is better than the Poo motor for running quality, but didn't notice much seat of the pants power difference at all riding them back to back through about 2' of fresh.
The XM does handle great in the powder now. You would have had a hard time giving me a XP or Rev from the ones I've ridden, but all the upgrade gizmos to the XM worked.
Slightly different turn in effort than the Pro or M (which I feel have very similar balance points carving and sidehilling), but after a few minutes of getting used to it I would say I was as comfortable on the XM as the Pro or Cat.