It all depends on your local utility costs per therm 1- Gas can explode and the fumes can kill you
2-Electric is clean energy BUt more expensive generally
3- Electric heaters can be had in 110-440 Volts
4- Gas can have various inputs as natural “gas” can have a low of 900 BTU’s per CU FT and a high of 1,200 BTU’s per cu ft. You have to contact your local supplier to find out your heating value. Also your height above sea level has a lot to do with gas heating Less oxygen less efficency.
5- Electric heater require no venting
6- In case of a black out you lose H/W not so with gas heaters.
7- To find the actual cost to operate contact your local utility
8-The “gas unit” requires more service as far as cleaning goes (flue and burners)
9- Electric heaters require a dedicated circuit BUT a water heater can go off a 3/4″ branch line DEPENDING on developed lenth and BTU input.
Both heaters depend on the watts/btu input for recovery rate