They say you save $ because not heating until you need it, but how fast is that meter turning once you are using it? What size would be needed for 1 1/2 bath, Kitchen sink, and washer for 2 adults and 3 kids? Electric on well water.

I'm not sure it's worth it for 2 adults and 3 kids, with electric to heat the water.
If you go online to compare, each model gives the minimum flow rate (in gpm) to activate the heating element, and the maximum gpm of output of hot water, and what it's good for in terms of all water using appliances and bathrooms.
The minimum is a pain, as is the wait for it to heat and then get started through the pipes.

Does your current one need replacing? That's a factor too.

Having the size water heater that's good for your family (50-80 gal?) while not being too big, and it being insulated and drained of deposits once a year, is probably what you want. And a GOOD on-demand one will cost. I paid $700 for a Bosch propane one, and that was on the low price end. A cheap one will be even more of a headache. (I have yet to convert from electric to gas because my electric heater, although too big, is almost new).