Sorry if any of this is a bit late in coming. Too much work and not enough play for me lately. If I may chime on this I would like to offer the following:
First and foremost any boat equipped with a diesel heater must also be equipped with one or more carbon monoxide detectors. This is not optional. Should the heaters exhaust become blocked, if there is a leak in the combustion chamber or for any of several other reasons then your cabin will rapidly fill with CO and you and your crew will fall peacefully asleep and never wake up.
Second, Amel has improved a couple of things over time with the evolution of the SM. My 1999 SM had an Espar heater and after two expensive repairs by the factory authorized dealer it would never run reliably for more than a few weeks. The intake was in the cockpit on the vertical "back rest" below the winches and above the large locker door/seat. It had a cover that one had to insure was open before running the furnace.
I had Amel install a diesel furnace on my 2003 SM using the fresh air ventilation system. They chose a Webasto as apparently Amel and I agree that the Webasto is superior to Espar. I'm not aboard and don't recall the model number but I do recall that it has the ECO fuel saving controller. Another improvement is that Amel installed the air intake at the bottom of the built in "sheet bag" (the area where the main sheet and other lines fall in to in the cockpit. The intake is at the bottom, port. It has the same bars across it as the older style intake has. This eliminates the need to have a door across the intake.
Finally, when heating my SM electrically (with recirculation air) moisture is a big problem and 2 dehumidifiers are needed to keep condensation under control. When heating with the diesel with outside air no dehumidification is needed as there is a continuous flow of fresh air entering the cabin.
If cost were no object I'd only ever run the diesel. But the single diesels cost in fuel is easily 5 times what it costs to run the 3 electrics plus 2 dehumidifiers, so anything above about 45 F/7 C I run the electrics when shore power is available.