Heater in a boat
 

I plan to install a heater in my 41' sailboat.
I understand that the two most known brands are Webasto and Eberspacher,
both German and probably almost identical. Any advice?
For example on the power: 2KW of heat or more? The boat is old S&S
design so that its volume is not as large as the modern ones and she has
three separate main ambients: forward cabin, saloon/kitchen, aft cabin.
Thanks
Daniel