Ken, dependent on [email protected]
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.