my best guess is it will be more efficient to heat the water. Water is a better conductor and more energy should go into the water as opposed to the walls and floor.

Maybe you can work out a way to heat the water in the greenhouse using the sun? There's some good threads on heating water badflash's bucket heater seems the most cost effective to me.

Tank Heaters?
My 700 gallon basement system

Also since this is an outdoor shed/greenhouse maybe you can consider heating with a compost pile?

brian