a liquid is without a doubt. you can run them at 90* temps and she'll never overheat. Liquid cooled engines tend to have more power as well, but they are also MUCH heavier. Fan sleds are nice, they lack some power, but on the other hand, they are very light. They go through the powder much easier than any liquid
Liquid cooled engines are much better except in conditions when there is not enough snow to reach the cooling plates. Liquid cooled engines can be built to much higher tolerences enabling them to get over 30 percent more power than a similar size fan cooled engine.
The only real drawback to a liquid cooled engine is that you need to have enough snow thrown up by the track to reach the heat exchanges to cool the antifreeze. Liquid cooled engines are not a lot heavier than fan cooled engines... and the horsepower difference between a fan and liquid cooled easily makes up for any weight differences in powder.
Fan cooled engines have a couple major drawbacks. 1. less power 2. has to be cold outside (at least less then 30 degrees) to keep them running at high rpm without overheating
In my experience, unless you live in a place where you get very minimal snow levels to ride on, a liquid cooled engine is almost all advantage over a fan.
i have a fan cooled sled. 1985 xlv 540. the sled is very light. and with a oversized fan cools just a good as a liquid. i can keep up with almost any sled around the same cc range. i have a few upgrades done to the stock heads and cylenders but i done it my self with a dremil tool. and the sled never overheats and hasnet had any trouble.
I dont have a lot of experience with fans but my father inlaw has a 97 polaris 500 fan, an it is such a dog, same with my neighbours bomby 500 fan, no throttle response. without a doubt a liquid is ten times the performer of a fan, I would never even consider one of these, after having driving these two POS's.
You can run fan cooled engines in warmer temps, you just rejet it. I've seen people throw fan cooled snowmobile engines in their ultralight 1 seater airplane before. Hell I've seen a free air Car before from the 1960s Ran great in the summer never overheated.