Yeti,
Are you looking at 2009 Polaris and Ski-Doo models? There is a big difference in the chassis of those sleds. The power is (HP) is similar, and both can be setup to respond even better.
I ride the IQ chassis and love the nimble response it provides side hilling and carving in the snow. I've ridden my buddies XP and it is a great sled as well. Better sled for pure hill climbing (not as nimble and tippy). He did have to put Fox Floats on the front to get it to carve the way he likes. The carburation can suck on the Ski-Doo (when you tip over, sometimes cold starting, etc.). The Polaris is fuel injected and that is nice because you don't have to make any changes for altitude (but you need to have re-map when you do serious mods). The XP is lighter and narrower in the front, with a wider track (16" vs. 15"). The Polaris is a little heavier (50 lbs or so), but holds up better because of it. I hit a huge rock this year that I know would have torn up the Ski-Doo bulk head. It's catch 22, lightness versus sturdeness.
The best thing you can do is ride both and make the determination on which one "feels" better to you. Also really depends on the riding you do.