Kevin,
"Energy density of a fluid does not correlate to efficiency"
You are missing a salient point and it isn't BTU efficiency ..what goes in vs what comes out ...However, with the significant diff in transfer rates, this dictates the size/cost of the heat exchanger to get the volume of BTUs moved to where you want them.
Home heating sytems that have air ducts vs water finned tubes can have the same efficiency in what goes up the flue vs what gets delivered to the house.
A circulator for a water system can be a very small fractional hp, but a squirrel cage fan run by a 1/2hp motor uses a lot of electricity ie 5oo+ watts to move that air around AND from my own experience ..the water system heats a lot faster so uses even less juice
BUT, the basic premise that you have only very limited utility during cold weather months was not denied, AND the equipment is comparitively very expensive for that restricted seasonal useage ...add that to your efficiency equation and the answer to the origanl question is obvious.
Stew Corman from sunny Endicott
(burning cheap local renewable firewood vs expensive Arab oil)