Be prepared to put in a larger electric panel if you are thinking of going with an electric on demand. The electric units draw a MASSIVE amount of current and if I remember correctly they need ~100amps of power to operate.
In my other house I have a Propane on demand unit and it works very well. The cost saving if there is any really isn't there. For me I needed massive amounts of hot water to fill the bath tub and didn't want to run multiple tanks to do so. I could put up some pics of the parties I had in that bath tub but I would probably be banned.
Six people could fit in there.
Over all I really like the on demand water heaters. You can stay in the shower for ever if you want and NEVER run out of hot water.
To the poster who said he was running out of hot water after a couple of showers you may only have one out of the two elements working in your water tank if it's electric. You could also have a bunch of sediment in the tank as well which reduces the usable amount of storage space for water.