Water heat: takes longer to have any effect, but final temperature is pretty close to what you want, effect can be reduced to below actual water temperature with a restrictor in the line, can be completely shut off by bypassing the water inlet & outlet. A serious leak will overheat the engine eventually.
Exhaust heat: takes effect more quickly, but final temperature is too much for high air temperature, more difficult to regulate or shut off. A leak won't harm the engine but will make the passengers sick.
