my money it would be the 600 e tec 2 stroke see lots original km well over 15k and several with over 20 before needing refreshing, with 118hp on tap it will power through slush and deep snow and will give you options on deeper lug tracks, as for overheating thats what studs and scratcher help with, if its low enough to overheat the engine then your damaging the sliders and track clips anyhow, 600 has better resale value as well,
skidoo has some sport models that use the older series 3 600 carb best deal on snow not sure if available with the utility sleds.
i ride a yamaha apex one of the harder to cool sleds and run near zero snow pack on the lake on regular basis dont have any issues with overheating, always little tricks to cool watter wetter, scratcher, studs nd another trick is to mount a single stud in rear of each ski throws all kinds of ice crystals into the skid