The big heater is like $1000ish and (this is just my impression) the heat cycle (cold/hot/cold) seems traumatic.

I may have formed this impression from the Sakura heater I had, it made really unhappy noises when it heated up and cooled down.

The Rinnai is quiet, but I always feel it's got a finite number of heat cycles before it explodes. If I can protect it from piddly demands, it'll live longer. Never mind it's 7 or 8 years old already and doing fine...

It also seems wasteful to run the heater for a minute to get 15 seconds of hot at the tap. A small dedicated on demand heater under the sink would give me an immediate 15 seconds of hot.