If you ask twenty peeps, you're gonna get twenty answers. I like the skidoo powerplant better, but almost everything else better about the Polaris. I believe that particular Polaris may have a wider stance which may make it a bit more difficult to sidehill. Polaris is easier and cheaper to work and maintain, parts are easier to find and much more common at least in Utah.
Do you do your own repairs? If not you may want to consider what dealer is closest or most helpful to you.
If your looking for used you will notice quick that there are alot of 6000+ mile ski doos out there on original engines. You may also notice most Polaris are in the 2-3000 mile range with "fix kits" or on their second or third engines.
Don't be afraid of a cat or Yamaha. The cats we found were in the 4-8000 mile range on original engines. The Yamahas were all fine they will wear the chassis out before the engine.
I found a cat xf high country 141 for a former Polaris guy and he loves it so far.
What type of riding, mostly trail, offtrail?
New or used sled?
Really aggressive or mild Mannered?
The Skidoo freeride will ride wheelies for days. Clutches create more heat than poo.
The polaris is quite a bit lighter. I think they handle hits a bit better than the Doo.
I think the engine in the doo is more reliable and smoother running.
I have not ridden a G4 850 but i know skidoo from 04-16 make me feel bull-legged with their wide tunnel and seat. They are more comfortable to sit on than Poo but Poo feels more like a bike while standing.
I feel the Polaris is much less squirrely and much more predictable , less of a learning curve in order to competently ride. Motor longevity is superior on the Doo. If you choose Polaris make sure you swap out the pistons every 1,800 miles in order to avoid the ring alignment pin failure that dooms Polaris top ends. It appears Polaris now has a nearly bulletproof crank since 2011, I've put my own Pros and Axys sleds through about 16,000 miles of mountain climbing since 2012 without any crank or worn out case issues.