A friend of mine services Bosch on demand water heaters and he has a few people who he has to acid flush their water heater every year or two because of hard water, so if you have hard water issues it might not be the best choice for you unless you don't mind flushing it, if that is the case then you might as well plumb in the valves and pump when you install it.
I also remember reading that some of the best on demand water heaters were around 60-70% efficient, cheap ones are worse.
Tank type gas water heaters that have forced exhaust tend to have less stand by loss because the flue is smaller and the fan slows down the convection, you can have as much insulation as you want around the out side but if you still have an open, uninsulated flue going up the center you are going to get heat loss from there, but a tight fitting fan assembly is going to slow that leak point, having a fan also allows you to put a timer on the tank because it will not turn the gas on without the fan (electric ignition tends to be standard on power vented gas water heaters) so if you both work 9-5 and sleep 10 to 7 you can set it to turn on and just heat a single batch of water instead of keeping a tank of water hot all the time, this saves you the stand by losses.
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