By a tankless heater I presume you mean a heater that only heates the water on demand. We in the UK call it a combi boiler as it combines domestic water with the centeral heating system. I had one in my last house run from L.P.G. gas as we did not have a natural gas supply. I found that it was very ecconomical and very efficient, there was always as much hot water as you needed and the heating system had to be turned down to about quarter what it was with an emersion heater in the tank. The house I am in now has a large gas heater in a cupboard which only runs about as much hot water to fill half a bath so as soon as I have the money I will get a combi again. The combi is also much smaller and fixes to the wall of the garage or utility room which will give me extra space for instrument storage
I hope this is of use to you
Bill