Anna,
The 220v heats faster, allowing a smaller tank in the heater, a smaller, cheaper heater, our tanks are less than a pint in capacity. All of our shower heaters are wall mounted and have the on/off switch and temperature control on the unit, which allows for a single water line to each. Because of my design I was able to use one on demand unit to supply hot water to both the bathroom and kitchen sinks. For those I used the faucets to regulate temps, again only one line to the unit.
If you are going to stick build you might want to look at some Australian housing/building/design sites, lots of useful info. Usable design ideas and the like. The Google International search engine helps get you there.
