Not necessarily. Although gas may be slightly cheaper, gas water heaters are more expensive than electric units and the venting now required for safety can be quite expensive. The thing to remember is that electric water heaters are nearly 100% efficient, as all of the heat generated stays in the unit instead of some going up the flue. Also, the Marathon is like a Thermos® in that it keeps the water hot once you have made it. It's the most efficient water heater made.
Many people are now building tighter, more efficient homes than they used to and indoor air quality is becoming a concern. Any fossil fuel water heater can be a problem with carbon monoxide, NOX, and other pollutants needing to be vented. You can eliminate this issue entirely, however, with the energy-efficient, electric Marathon.