Thanks guys for the response. Sorry I did not get back sooner. Work has been crazy the last two weeks.
dgbldr, I will get my profile filled out, thank you. I had a hard time getting logged on and did not realize it was not complete.
As far as the house size, it is about 4000sf with a master bath that has dual heads and a 80 gallon tub. The tub does have an inline heater.
There is a guest bath and an upstairs bath. The home is only occupied by two people but it is the filling the tub that I am concerned with. There is natural gas there. They are currently using two 50 gallon gas fired.
It seems like the responses were about half for on demand and half were for tanked. Nobody has had trouble filling a large tub? Has anyone tried the hybrid tank/tankless by Rinaai