I was using 2 ceramic heaters for the first couple winters, they did okay, but had trouble keeping up when it dropped below 10* (in the original 12' X 12' section)
Used this heater last winter, it's a 240V, 4000 Watt unit. Really cranks out the heat, also cheaper to run than the other two heaters combined.
Natural gas/propane would be the least expensive to heat with, if you are looking to just keep things above freezing, then one or two small electric heaters should work. Also make sure to have a backup heater available if you lose power.
Caulking your GH as airtight as possible will help reduce the heating load, you can also use pool covers, bubble wrap, etc, to help insulate.
(Lowes sometimes has these on sale, here's a link to the catalog:
http://www.lowes.com/lowes/lkn?action=p ... lpage=none)
Steve