Depends on a lot of things.
The big ones
1 Rider style
2 What you know and if not much, Dealer support.
This year they where all a lot closer than in years past, I can honestly say they all have strong points to the point one will do something the other wont do.
IMO, I'm a backcountry rider not so much a climber anymore so here is my opinion.
The pro is the best chassis for extreme backcountry riding, its designed specifically for it. When it gets to the really steep it will hold a steep line and maintain more speed than the others the steeper it gets. The Cat comes in next but nearly equal to the doo, the doo can do it but its slow until you get the running boards out of the snow, the doo is smoother though.
The cat is a great all around and takes mods the best, very capable and 95% of the time or more will do what needs done.
The XM definitely pulls the best wheelies, is a blast to ride does it all well. Fun factor is big because its so easy to have fun. But it wont hang on a real steep sidehill like a pro. Although it holds some steep lines unlike the XP and is very smooth, it slows way down when its that steep and a pro on the same hill will gain speed and be able to maneuver a little more, to most its not even a factor, for me its a huge factor.
Like everyone else, Today its about you not me, all the sleds these days have more of a fun factor than ever, and in climbing I've seen all of them win in a straight up climb depending on conditions (all setup well) so thats not really a factor IMO.
I would go ride them all, I had fun on all three. But all three in stock form also had an advantage in one place or another, so many variables its not even funny.
I'm leaning toward the pro or cat, only because its a lot harder to modify the doo for what I like