I answered a similar question on SE Sustainable Living recently.
Before anyone (including yourself) can answer your question you need to supply some more information.
The first thing you need to find out & provide is, what is the heating value of the gas. In metric units this would be the mega joules (MJ) of heat that one cubic metre of gas produces. In US customary units it will be the amount of BTU (or btu, being British thermal unit) per cubic foot of gas, or the BTU per Mcf.
Energy companies charge by the MJ or BTU value of the gas they supply because different types of gas have different energy values. Even though natural gas is methane each reservoir of gas has a different composition of natural gas, i.e. 98 percent methane and 2 percent other gases.
The other thing you need to find out is how much heat the electric heater will produce per kilo watt hour (kWh).
To compare between gas and electricity you will have to convert either the gas heating value from BTU/Mcf to kWh/Mcf (if your using metric units J/m3 to kWh/m3). Or, convert electrical BTU/kWh to BTU/Mcf.
The other things you will need to know are the costs for gas and electricity. The cost of electricity will be in dollars per kWh and the cost of gas will probably be dollars per BTU, but it also might be dollars per Mcf. Which another reason why you need the heating (or energy) value of the gas that could be used.
Having this information you then do a cost comparison between the two.