Building a custom house; owners want a back up-generator, so I am switching gears from Electric mini-boiler & Marathon 105 gal electric H2O heater to LP systems so my generator can run the entire house (Power load will greatly diminish w/ gas).  I am very familiar w/ electric systems. But know very little first hand about gas.  My Plumber says to go with a 50 gallon Vertex w/ a double flat plate heat exchanger and my HVAC sub says to go with a Navien Combi water heater.  I have Hydronic in-floor heat needs as well well as domestic hot water needs.  Slab on grade house 1600 sq ft w/ a 24 x 24 attached garage.  Located in -western WI.  I trust both tradesman and know they specialize in hydronic heating systems, and have worked with both parties previously.  The house will be a second home for the owners - I really have no idea how much time they will spend in the house.  I do like the idea of the Navien because we can tie into the Air-handler and air source heat pump in the off chance the something may happen to the in-floor heat system.  Any one have opinions on either system  (Navien or Vertex) 
Thanks WI HANK

I would almost always prefer a tank to a tankless in a heating application. The vertex is good. HTP Phoenix is good too.

Rob is right,

But don't forget the 24 year old Polaris condensing water heater. Nearly all of my small home designs now incorporate a condensing water heater and SINGLE-WALL heat exchangers. The question also comes up about plumbers servicing high tech equipment like the Navien. It is not in any plumbing training course.

The ICC allows single-wall heat exchangers, as do the state plumbing codes of WI, MN and most others. Double-wall heat exchangers are almost NEVER required or needed in residential applications, while they ALWAYS waste electricity and fuel.

What will happen to the radiant floor?

I find most redundant air handler hydro-coils unused. More of an expensive crutch for non-believers. A mini-split is my choice on most radiant floor designs. Plenty of back-up, with much smaller emergency generator demand.