are you talking forced air or hydronic? After looking at all the choices I installed an ITR Hurricane hydronic. It heats water which is circulated to heat both the boat and domestic hot water. It is a fairly sophisticated system with a lot of options to think about. Mine has 4 zones for the heat exchangers, plus a heat exchanger for domestic hot water (very quick to heat and unlimited hot water), plus a heat exchanger for the engine so use waste heat when it is running. One selling point is the low pressure jet (less clogging is the claim) and significantly lower temperature exhaust. It it pretty quiet too, some of the Espars sound like a jet taking off. The build seems to be high quality.

http://www.itrheat.com/Hurricane.html