I would suggest going with some sort of progressive setup. That way you can add more water as the truck works harder. If you use a simple on/off you only get water near the top where you do not spend much time. Also when empty you will hardly use any water. I set mine to start at 6 psi and full at 16. Empty I almost never use water except for the quick sprint to get on the freeway. Pulling will be another story. I have not yet hooked up to the trailer(s) to see how much water I will be spraying.
I went with cooling mist and am happy. Their customer service is awesome.