OK, I've installed three hydronic systems, and two forced air systems on my own boats over the years. These were "from scratch" installations.
The systems have been Espar, toyotomi, and Wallas brand.
Hydronic is much more complicated to install. You need to route two heater hoses and DC power to each heat exchanger.
You also have to design the system so that you have a coolant tank at the highest point. You also have to design in several valves so that you can use a separate pump to pump coolant through the system.
You also need to design or purchase an electrical interface system to add zone control to the heat exchanger motors.
Startup can be a bear, getting all of the air purged from the system.
The advantages of hydronic are that since the hoses are 3/4" instead of 3" air ducts routing is easier. Building in zone control is easier because of this. You can use the hydronic to heat up potable water, another advantage.
Forced air systems are much less complicated to install. They generally come with the electronics/thermostat as part of the kit.
With a forced air system, you can utilize outside air for a portion of your air flow, providing ventilation to your boat.
The challenge with forced air is zone control. On a larger boat like our 47 we need heat allot in the cabins, less in the salon, and even less in the pilot house. Forced air systems are fairly limited in their BTU output. There are larger forced air systems but for the most part they are limited to around 10K BTU per unit.
Both units are very noisey for the most part. They sound like a turbo when outside the boat.
On our latest boat a 47' pilothouse we went with Wallas forced air furnaces. These provide both zone control and ventilation.
The Wallas units also have an advantage of being very quiet. You cannot hear them running. Another advantage of the Wallas units is their new thermostat. Instead of a simple on-off their thermostats modulate the flame so that you get just the right heat output for your situation.
We went with a 10KW unit to serve the cabins, a 10KW unit for the salon, and a 7.5KW unit for the pilothouse.