Don't get more on-demand than you need. Smaller units for 1-2 bathrooms are more efficient than the larger ones that are sized for larger houses.
There are two reasons tankless heaters can be more efficient. One is they don't heat water unless you're using hot water from a faucet or appliance. The other is that they only heat the water as much as you set it for, meaning it's a variable output burner. This heater is 199K BTU max input so make sure you pipe it for that possible demand.
For comparison, my Rinnai is 185K BTU and it supplies 2 baths, a kitchen, our radiant heat. One nice side effect is you can fill a hot tub, that's the endless hot water thing working in your favor.