Are you looking to buy new?
If so, the 600 E-tec doesn't add much to the price, gets about the same fuel mileage (yeah, amazing huh?), and has a lower maintenance schedule. AND, it cranks out double the HP.
You're right, people tend to think bigger is better, but in reality, most riding isn't done at WoT. I drove a Touring 380 for years and years (still have it) and the only reason I ever wished it had the 500 was deep powder snow. However, I recently replaced the old comet clutch with a newer TRS on that thing and it doesn't bury itself anymore in the deep stuff. It slows down, but it keeps going.
I've been looking at buying a new Skidoo and looking close at the motors. I'm really liking the specs on the 600 e-tec. Double the HP of the 600 ACE and it still gets 21 MPG vs the 600 ACE's rated 28 MPG.
600 Ace = 60 HP
600 e-tec = 120 HP