25 years--probably 30--of guaranteed low performance or 15 years of stellar performance if sized, installed and maintained properly. One with with an open chimney and the other eliminating the chimney and it's energy draining perpetual draft, the other with an ecm modulating blower, burner fully condensing with built-in outdoor reset.
Boilers warranties have no real value and are in no way a indicating factor relative to reliability, durability or serviceability. Things you really want.
We install Bosch combi boilers with aluminum heat exchangers and a 5 year parts & labor warranty. We also flush, clean and treat all of our residential and commercial boiler systems ensuring long heat exchanger life and better system thermal and system efficiency.
If you have cast iron radiators, a condensing boiler is a perfect match as the radiators were designed for relatively low operating temperatures. We do not use the SlantFin condensing boilers on single-zone gravity conversion cast iron heating systems in old homes, since they have a warranty enforced mandate to add a redundant pump (primary/secondary) where it clearly isn't needed. Baxi, Bosch, Burnham, Crown, HTP, IBC, Lochinvar, Navien, Triangle Tube and Viessmannn have no such requirement.
As for as not living there forever, you can sell your low fuel bills with the house.