Now as to make of Tankless water heaters, I have to say Noritz is by far the best out there. It is more advanced than the Rinnai, comes with parts that Rinnai does not like the advanced controller. Now Rinnai is a for off second choice for me, then even farther off third is Takagi, I would not even consider any of the others.
Lets compare the Rinnai R75LSi / GE 75DNSRSA to the Noritz N-0751M-DVC
Noritz 11,000 - 199,900 Rinnai 15,000 - 180,000
Flow rates / temp settings.
Noritz .5 gpm - 9.8gpm 100º - 180º Rinnai .6 gpm - 7.5 gpm 98º - 140º
Noritz can be set to 180º with out any special controllers, the Rinnai needs a special remote that needs to be purchased. Also Rinnai's min flow rate can vary from .6 gpm to 1 gpm
Noritz uses 100% stainless and is adjustable. Rinnai uses galvanized steel or plastic and requires cutting to adjust the lengths.
Noritz heat exchanger is 25% thicker than the Rinnai's
Noritz uses a duel flame burner which gives a better temperature stability. Rinnai is still using a single flame burner.
Noritz comes with a controller that gives you full diagnostics. Rinnai must purchase a special remote.
Noritz allows you to make a simple elevation adjustment by just clipping two wires together, the Rinnai you need to set the dip-switches. Noritz also has a built in controller for recirculation pumps where the Rinnai does not.
The one thing they do have in common is the warranty 12 year heat exchanger/ 5 year parts/ 1yr labor