Heat is the enemy of any engine and a liquid cooled engine will generally run cooler than an air cooled one and this is especially true during slow going trails and when doing farm chores such as spraying. I leave my engine running if I need to get off to do something and the thermostat will kick the cooling fan on and off, an air cooled engine can't do this and can overhead. My experience has also been that a carburetter air cooled engine does not run worth a darned until you can get some air going by the fins and cool it down. Another consideration is heat output to your legs. Any air cooled ATV I have ever ridden is much more uncomfortable than a water cooled one. I am not an exert and these are only my opinions.