Trust me you don't want a tankless.
I've installed dozens of them, not only are they gas hogs,( requiring more gas then the average house) they are only about 80% eff. and you still have the volume of the pipes before you get warm water.
I have a recircilating system now. IMO thats the best bang for the buck.
If I wanted a green system, I'd go with a hybrid solar and tank unit. The solar handles all but the last 10-20 degrees. which the tank handles.