for 100,000BTU in electric, it would cost you around $4.27; for 100,000BTU in gas (one therm) $1.06. Now, with an electric WH, you get almost all (not quite) of the energy applied as heat, but with NG, some of it goes out the flue and doesn't heat. A low-end WH is about 60% efficient, some are over 90%. It would have to be about 25% efficient to cost the same as electric, and none are that bad.
Plus, a NG WH will recover quicker than an electric one, at least most of them.