For 2 showers a larger Takagi or Rinnai are good. It is all about gas
supply, water gpm, incomming water temp and temp rise produced, you have
to measure everything and not guess. I don`t know your location but
incomming water temp lows are most important in your pick, my water goes
to 35f and I do fine with a small 117000btu Bosch, actualy I never set
the heater on high and get a hot shower, but 2 showers probably needs
185000 btu. Another over looked issue is gas supply, a tankless will
likely more than double or tripple what you use so calculating supply
with all competing apliances on and gas pipe size is critical if you
want your tankless to output its rated Btu, if it doesnt reach it rated
btu you can be left cold. You need to measure flow with a manometer.
The payback is there, for me its 4-5 yrs, but if you have a family you
can loose money if people get the idea the HW is endless and have
endless showers. The greatest payback 25-50% is the single user.
Tankless have nothing to rust, should last 30 yrs and don`t loose
efficiency every year, as gas tanks do, due to scale. My old tank had
1ft of scale in it so it was probably 50% efficient. If you have the $
takagi makes a condensing 94-96% efficient Propane tankless. Look into
the remote thermostat option.
I installed the Paloma 7.4 GPM unit available at Home Depot and am very
pleased with the performance. With the energy tax credit and utility
rebate I received my actual cost was less than a traditional water
heater. What the other guy said about gas flow etc. is all valid and I
would add that when in doubt about sizing the unit - go bigger.