Natural gas is cheaper, is available on tap (in many places), is less sensitive to cold and produces less CO2 for a given amount of heat. It is also easier to make sustainably.

LP can be installed away from the 'grid', can be portable, has a higher calorific value per kg (irrelevant really :), has a more colourful flame. Possibly more GH friendly heaters available, depending on where you are.

You can look at my link and see what I do. I use natural.

Here is a link that might be useful: My Greenhouse

If you can get natural go with it. It's cheaper in the long run and you don't have to worry about running out at the wrong time.
Wish I could.

MollyD

Given the opportunity to choose, go with Nat. Gas. No contest.

Nat. Gas prices are cheaper and far more stable than LP and neither rain, nor snow, nor wind nor massive traffic tie ups as result; halts the flow and delivery of it. Nor has anyone ever failed to receive it in the worst blizzard because a delivery truck ran into a ditch, or broke down.

Of course, if some fool who can't read UG line maps hits the gas line, you will lose your heat for a while, but short of such a sceinero starring a brodacious backhoe, or a massive earthquake that rips lines out of the ground, as long as you pay your bill, your natural gas will just keep flowing.

Virtually all gas heaters sold in the US are made to operate on either Nat. or LP and the prices seem to be the same for either.

We live where ice storms too often take down electrical lines, but as long as we plug our Nat. Gas furnaces into small, stand by portable generators to operate the fans, we always have heat. Oil lamps and candles to see- but warm.

I went with a natural gas 20000 btu heater today. It is a convection blue flame and a radiant in one. Only one I could find in my price range. It should be in by the end of the week. I will let you guys know how it goes. Now to rent the ditch witch to bury the gas line, water line, and electic...... wish me luck!

Oh darrinky, your fun is just beginning..... :-)