550F
Cons: poor gas mileage, 2 strokes usually need to be rebuilt much sooner than 4 strokes, use 2 stroke oil, if oil injection pump dies engine seizes.
Pros: 2 strokes are lighter, fan cooled better on hard pack and ice especially in warmer temps, start better in -30+ temps, 2 stoke engines are easier to work on.
600 ACE
Cons: Heavier due to 4 stroke engine, need to change oil and filter like a car, harder starting in -30+ temps (need block heater, batt blanket and plug in), over heat in warm temps and hard pack, glare ice conditions, 4 stroke engines are more difficult to work on.
Pros: Excellent gas mileage, no oil injection pump to worry about, are quieter and don't smell like a 2 stroke, very good low end torque and smooth power through whole throttle range, much longer time between engine rebuilds.
For warm weather running with a liquid cooled sled install a set of these (http://www.yamaha-motor.ca/eshop/detail.php?sectionId=AC&groupId=19&subGroupId=34&categoryId=105&itemId=2253&unitId=2174&unitName=2008%2BRS%2BVenture%2BGT&unitGroup=SM&unitYear=2008&items_per_page=12&page=8) and the problem is reduced to almost nothing.