Steelman - I just got back from Idaho last week where we had two M1100T's, my Pro RMK and the PC 800... Here's what I think: it all comes down to what kind of rider you are and where you like to ride. For reference I am 5'9 and about 185 lbs.
I spent very limited time on Hilly's M1100T, but we both agreed the vertical steering post upgrade is a must. That was my second thought.
My first thought was that the sled hauls the mail. Period.
As far as being able to sidehill it, it's absolutely possible. After a short time I could whip very tight circles on it on the flats, and it pulls onto its side easy enough when you get going vertical. It was a very fun snowmobile in more open terrain. Plus, the low maintenance is awesome. If absolute HP was my #1 requirement, there's no doubt it is the snowmobile I'd buy. It's not ever close.
However, for me, power is not my #1 requirment. I rode Cooke and IP this year and there was not one place my RMK (or the PC 800) didn't have the power to get me. For me, my #1 requirment is weight, #2 is how easy the sled pulls onto it's side and #3 would be horsepower. I don't run any big chutes where I need 200+HP. My favorite riding is short, steep tree-filled climbs, technical sidehills and smaller jumps (no cliff drops or anything like that). That's why I bought the sled I bought.
My advice would be to take an honest look at the type of riding you enjoy the most. Once you know that for sure the choice should be obvious.
BTW.... You can't go wrong with the PC 800, it's a great sled.
Joe