I've been on or around pretty much all of them. They all have their pros & cons.
I started with sport quads, light stupid fast very smooth ride & pretty much useless after that. My last estimate on invested capital on my '03 Z-400 is somewhere in the $12,000 range (factory price of ~$5600)... that was a good idea
But power wheelies on the pavement @ 55mph is always fun. (2 concusions & 2 broken ribs & a fractured wrist later). I still have it but it doesn't come out much.
My second machine is an '07 Suzuki Kinquad 700. 2" lift, 27" Swamp Fox tires.... Much more versatile
Somewhere in the neighborhood of 3K miles on it now & still kickin butt. My father has an identical machine & they are both damn good. Cons, battery will die if stored for too long which sucks ass- No power steering which makes it difficult to operate in rocky/rutted terrain but still manageable. Pros- The KingQuad will out run the 700 Grizzly (my uncle ran the Grizz)- They float!- it's been rolled over backwards twice & keeps on tickin... - We use our ATV's to pull custom made trailers into our hunting area & they handle the extra weight (read overload) very well- The KingQuad also comes with the pull rope starter (thank god), some quads don't.
Newest addition is a 2013 Polaris Ranger 900xp. 1000lbs bed capacity- 2000lbs towing capacity, bench seat & the new 900cc engine will push speeds up to ~70mph. WAY quieter than the Can-Am Commander & a hell of a lot more room.
My folks just picked up a Can-Am Commander 1000. The Rotax motor is a friggin beast, serious power. Noisy as hell, both the drive train & the motor. Power steering, I really don't like the feel of the powersteering, it's kinda like steering under water; maybe it'll be better in the rocks. Lots of heat in the cab, they were cooking in there while riding yesterday (family ranch).
Most of my family runs Polaris, for the most part they're good machines. Plush ride & ergonomically friendly. Usually underpowered & don't perform with the imported machines. The twin cylinder machines sound like Craftsman lawnmowers, I can't stand it. The starter always sounds funny but functions well.
I've rode thousands of miles via ATV & can tell you this. There is NO such thing as too much power. I've seen far more people get in trouble when there itty bitties power out than I've ever seen folks get in trouble from too much power. YMMV