Regardless of the type of heater you decide to install, your boat should be as well insulated as you can make it. In addition to having Pacific Seacraft insulate our deck and hull (down to the water line), we added 1/4" and 3/8"
closed cell foam to the inside of hull and deck surfaces that were exposed to the outside. We also put expanding foam insulation in the cavity left by the hull deck where we couldn't reach it to push a half-cylinder of closed-cell foam up to cover the wiring and seal off the hull-deck joint cavity. We glued acoustical wall covering to the inside of the V-berth where it was exposed--it's better looking than closed-cell foam. Without the insulation condensate dripped down inside lockers.
Friends who own a PSC 37 showed us how to make a clear plexiglass "curtain" that snaps onto the side of the cabin trunk, forming a fitted cover for all of the portlights at once. You can still open the portlight and let in a bit of air, but condensation is cut to almost nothing.
We have an Espar, but it had a hard time keeping up with the cold winters in Washington State and Canada until we installed the insulation. The Espar ran at least 50% less after we got all the insulation installed.
If you haven't read "The Warm Dry Boat
" we recommend it. The most important thing we did (described in this book) was to be sure we had adequate air circulation. We used a combination of slightly open portlights, hatches, and cabin fans to keep the air moving. Even so, if you're at the dockside in a cold, wet climate you may find you need a dehumidifier to make life bearable. The majority of our condensation problems were caused by cooking--opening the hatch over the stove helps, but there is still a lot of moisture to get out of the boat.
Definitely look at electric heaters if you're at the dock and have dockside power. We used a Caframo floor model, but friends swore by one that heated oil in a radiator-like free-standing job.