Jake53...I can't answer your question about how long a tank will last but I can talk about the propane deal as I have one in my shop. Mine is a Lennox propane that I hooked directly into the BIG tank I had for the house. I use the term "had" because I moved a while back. The unit did a fabulous job. The shop at that time had no ceiling or insulation but would still be comfortable to work in after about 5 minutes. Now understand the weather here in S.C. isn't real cold during the winter but it does get nippy. Now that I have moved I have built me a 30x40 shop and I brought the heat system with me. I plan to hang it in the next week or two. I still have the ceiling tile/insulation to put in. I put the suspended ceiling up myself and with the insulation I know the unit will do a very good job. I plan to buy me a 90 gal tank since there is no gas line where I now live. I know the unit will be nice and I don't think it will be terrible on the pocket book as I didn't notice a big increase in the usage before. (the price is what increases!) I love the quick heat and there is no pilot to stay on as it has an electric ignitor and wall thermostat. I love it. I don't know about you but when the temp is around 55 its just right to get work done. I think the whole thing depends on how "hot" you have to have it in the shop. Since your walls are insulated you should make out very well.
I know this didn't answer the question but I do vote for the gas heat!!

1937 Chevy Pickup
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1950 Chevy Coupe
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