Does anyone know of articles, studies, etc. that will help us decide between these 2 options?
1) hot water recirculation (retrofit kit for existing pipes), e.g. the Watts system at Amazon.
2) on-demand water heater(s).
I would think that the latter (#2) would require 1-2 units at each of the points of use for the water distro pipes of a house. 1 for each shower/bath tub, faucet (e.g., clothes washer) and 1 for each of the sinks. That's what we had in Japan (and Germany) back in the 1980's when we lived overseas.
On the other hand, the former (#1) could add to the overall energy use/inefficiency of the home, but would reduce the water wasted each time you turn on a shower or sink out at the furthest points on the water distro pipes. And I've even heard it can also reduce the amount of overall water heating inefficiency (but have not seen any data).
We have a 1956 single-story ranch in an 'L' shape that's built over a crawl space. It's hot water heater is a tank and is corner of the L. The kitchen is at the end of one leg, and the master bathroom is the end of the other leg so one has to turn on the hot water and wait at either/BOTH locations for the sinks, showers, etc. to get hot.