If you're concerned about the environment, electric water heaters are the way to go. Electric units are more efficient but will likely result in higher monthly bills. Gas heaters use more energy to operate and emit more waste, but since natural gas is less expensive, monthly bills will likely be lower with a gas unit. Upfront costs on both units are about the same. However, if you already have an electric unit, switching to a gas heater will require gas line installation, and that will require more money.