Unless your water heater has a horrendous latent heat loss, the cost of heating the water should be the same whether it is heated and stored or heated as needed. Also, a tankless heater has a finite flow range. With just the two of you, you should not exceed the maximum gpm, which is usually about 6 gallons, but it will also not heat water if the flow is less than 1/2 gpm such as a gentle rinse application.