Well, as the title says, What is your opinion or what has your research turned up as the best, or most efficient way to heat your greenhouse.

heat/cool the water ??
heat/cool the air??
heat/cool both??

I am in the process of final plans on my greenhouse. I started out with the idea of subterranean air movement for h/c transfer. I have since, been giving it second thoughts. I am now concentrating on h/c the water. I believe this will be a more efficient use of electricity and the water will make a better, longer lasting h/c sink, than the air, which has no mass to act as a sink. I also found the air ducts were taking up valuable space in my Greenhouse, where space is at a premium.
I have my own track hoe, so either way is readily accessible, I have just decided from my research that my money on hardware (pumps plumbing etc.) would be better spent insulating the foundation better and digging a trench for subterranean water cooling and relying on solar heat in the winter. Both from heat trapped by the water in the grow areas and fish tank, and from a circulating solar panel heating system
I am certainly interested in any ideas some of you have from your research or experience, whether it backs my idea up or disputes it, I am interested. My design changes will be cheap now, they are on paper, one month from now, they will be more expensive, they will be in a building. Much more expensive to change then!!
Thanks for your input