Which water heater should I buy (to save the most)?

I have an oil burning furnace and oil fired water heater tankless coil inside it that supplies our hot water. I want to replace oil heating with pellet stove and then I heard the oil fired coil will not be economical because the house heat will not be in use. I would like to get some kind of tankless gas heater or electric or natural gas but don't know which one is the best. Our family does not use much hot water (about 2 showers a day and 2 loads of loundry a week). Anyone knows what is the best in this case? We live in upstate NY.