depending on your water pressure, some places like ours need cold and
hot turned on for sufficient flow at the upstairs showerhead. at our
home we only have 42 psi from the city water coming thru a new one inch
main.
you may want that hot water at 140 degrees for you and the dishwasher.
this means a rise from cold city water 105 degrees at whatever gallons
per minute you have. whether any tankless unit can achieve that depends
on the entire home system for gas, water pressure psi, water flow gpm,
personal preferences, and future owner needs. i can't imagine we'd
enjoy waiting for the tub to fill with barely hot water more slowly
without the cold flow that speeds up the process. we use a regular gas
75 gallon water heater.