I just went through this decision process and decided on the propane bulkhead heater in my case, but I can certainly understand why other choices would make sense in other cases; there's some pretty decent products out there for each of the different fuel types.
A few factors that swayed me . . .
1) I have a propane locker in the back of a canoe stern boat and since it's advised to vent a diesel exhaust where there's no risk of it being below the waterline when heeled over, such as on the stern centerline, I couldn't run a hot exhaust pipe through the propane locker, so this was problematic.
2) For long range crusing, I didn't want to draw off the my diesel supply in case it's needed for running the engine. I can use 2 20lb propane tanks on my boat so I can use the LPG for heating / stove and reserve diesel for the engine.
3) I like propane for the reasons stated by others above, but also agree that the precautions mentioned are really important, such as having a fume detector and a CO detector, and also performing the leak checks frequently.
4) The cost of the propane bulkhead heater is significantly less than forced air diesel heaters.
5) The forced air diesel heaters use a bit more battery to run the blower.
6) The simulated fireplace is a nice touch.
These were some of my reasons, but I agree there's some offsetting benefits for some of the other options that can make another choice better depending on the boat, user requirements, and budget.
I agree the duel intake / exhaust pipe is a real plus for the bulkhead heaters, so there's little risk of consuming all the oxygen in the boat and/or having the exhaust become a problem inside.
I share the concern about the 3" hole required in the deck. I'm considering installing a larger deck place first, and installing the intake/exhaust pipe into that, so if I were ever in really dangerous seas, I'd have the option to remove the pipe and substitue the deck plate lid. I haven't thought this one all the way through yet though, so I'd be interested in any feedback anyone might have on a better approach. The concern about getting lines tangled around it can often be mitigated by a guard the vendor offers.
I did an install of Dixon P-9000and you are invited to read my post:
Sail Delmarva: Search results for heat propane
3" hole in the deck.
No troubles yet, and I had 30 inches of melting snow at one point! Do seal the cuts with epoxy. The stack came with a good gasket and I added 3M 4200 for luck. the stack had no trouble melting a hole through the snow, though I did check on it now and then.
Heat on the deck.
I can only speak for the Dixson. Because of the double wall pipe, the external pipe is only about 140F where it aproaches the underside of the deck. The sleave that touches the deck is only warm to the touch - you can hold it.
The Dixson design seems impervious. I have had 45 knots without trouble. Lighting in those conditions is a bit more touchy - close the door fast - but not difficult. I also added a solid spray guard forward to deflect green water.
Try electric heaters first, to be certain how much heat you need. I am an engineer and know how to calculate heat loss... but how do I know for sure what is in the walls and where the leaks are? I prefer test data.
I made a custom guard that does fine. It also keeps lines away from the hot areas (would'nt want to damage a line). Do pick a deck location that is not underfoot. There should be a no-snag cure for any stack.
There is a fan, it's noisy on high, and so I generally turn it down. I only need high to warm the cabin, after which I turn the gas and the fan lower. The power draw is about the same as an anchor light. It will run without the fan, but the fan really helps spread the heat.
I have a cat and this is a real problem. I use fans to chase it around, but the sleeping cabins are aft, around a corner, and stay like ice. Some boats really need central heat, particularly for live-aboards. I think cabin heaters are for occational cruisers, like me.
I added a CO monitor and it has never chirped. I have fume detectors and all of the standard propane safeguards. I like the fact that the Dixson is sealed from the cabin, further reducing potencial hazards.
The stack temperature when at full load is 310F with 6% O2 in the stack, giving an efficiency of 85%. That is as good or better than most boilers, better than most all small heaters, and as good as you are going to get shrot of and ultra-high efficiency condensing system, not available on boats. I have heard some coment that the Dixson design is wastefull because it does not give heat from the stack. Not true; the combustion air is pre-heated and no cold air is pulled in under the door. All good, because propane is ~ $12/20 pounds for me.
Fire place look.
Nice. My daughter has used it for marshmallows!