1028 BTU = one cubic foot of natural gas.
1 BTU = 0.2930711 watt/hour
you need to know your energy costs. Gas is usually per 100 cubic feet or a therm, which is close to that (don't know the conversion).
An electric is close to 100% efficient; a gas-fired system will lose some heat up the flue - anywhere from 50-90+ % efficient, depending on the design.