Personally, I would Go with the boiler,
I doubt that the tankless
would actually last 20 years..
the tankless will give you fits with the return
recirulaiotn line run..
and I would gamble that the tried and true boiler is
about the same price or less than a compairable tankless..
I am wondering
why not just another 100 gallon commercial heater
what is the reason for not doing that???????
2 75 gallon gas heaters is an ok idea too but you will probably
be wearing out the heater that heats the floor every 5 ;years or so.......