I have decided to eliminate my floor-standing, 1500W heaters (other than for backup purposes). I have found a ProFusion Heat, model EH-4604B, that is a ceiling mount. Northern Tool & Equipment sells this unit (as well as other companies). It is a 240V, 7,500W forced air unit that produces 25,600 BTU at the 5,000W setting. I can also use a 3,000W and a 4,000W setting that would produce less BTU. I imagine, and this is just a guess, that I would probably use the lowest wattage for the vast majority of time. The 5,000W setting is rated to heat a 1,000 sf. space with a 12' celing. My greenhouses are only 120 sf. with 8'-9' ceilings. Does anyone have experience with this particular unit?

Ken, dependent on K@Hr rates these may be alright, esp. if propane is expensive in your area. You'd have to figure both rates/BTU. They may be be a great back-up in my area, since weather can effect supply, demand, and availability of propane. If they operate anything like our heaters of the same design used in ceiling-less warehouse cubicles, good air circulation would be paramount in preventing radiant scorching.
Since my propane is independent from the domestic power and the cost/BTU is almost equal, I use gas. I often have my electric set as back up. In the event I have trouble getting propane I set the electric as primary and save the gas as backup.
In summary, if your cost would be less with the elec. Go for it, esp. since if you have power to the heaters you will have power to your fans. You could retain your gas for backup. Depending on your supply source you may want to invest in a couple of 100# cylinders to avoid standby charges.
At the end of the heating season we wait till July or August to call for a tank fill to avoid standby cost.

I have those two, 240V heaters installed. I decided to go with the models that can either produce 3000W or 6000W. I doubt I will ever use the high setting, but time will tell. Perhaps only using the low setting will extend the life of these heaters. I am leaving one of the 120V, 1500W heaters (one in each greenhouse) on their own dedicated thermostat and they are on another electrical panel. Those will be my backups just in case the 240V trips a breaker or for some other reason, don't heat. I hope the 240V will be more efficient and perhaps use less electricity. These heaters, and running those 240V dedicated lines, were not an inexpensive venture.

Only time will tell.