I think it all depends on size, and how much above outside temps you want to keep it.
I only aim to keep mine above freezing as I have some very cool loving orchids out there. I use a little electric 200W oil bar heater with a tiny fan heater if it get's even colder. It's only a 6' x 6' space, I add extra insulation in the winter and I'm not aiming to keep it that much above outside temps (in fact my target minimum temp is lower than the average winter temps here).
For me that works, has the backup of two heaters in case one fails and does not cost too much... but I would expect it wouldn't be the same for a larger space and for keeping it higher about outside temps.
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