I finally decided to replace the water heater I have with a tankless one. After some research, I narrowed the choices down to two "finalists": Takagi and Rinnai tankless water heaters. Rinnai seems to have some advantages, among them higher output (8.3gpm vs. Takagi's 6.9gmp) and, most important, lower kick-off flow (0.5gpm vs. Takagi's 0.75gmp). On the other hand Takagi seems more spread than Rinnai and also I could find a lot of information for Takagi and almost none for Rinnai.
Need some advice from people who are using these water heaters.