Should I get a condensing boiler or atmospheric boiler for cast iron radiator system
The ONLY argument in favor of the inefficient boiler would be IF it's so basic that it will run with the power out, and thus might help prevent the house from freezing in a long winter power outage (though that would be a limited level with no power for the circulator pumps.)
Otherwise, efficiency favors the condensing boiler. If long winter power outages are a thing in your area, a gas-powered (the same sort your boiler presumably uses, not gasoline) backup generator sufficient to keep the boiler and pumps and fridge and freezer operating would be a solution that does not require throwing a bunch of heat up the chimney, and has more side benefits in the form of the fridge and freezer still working, as well as some lights.
If the chimney is suitable (it may not be) you can get the &quot;not freeze house&quot; and &quot;operates with no power&quot; functions from a wood-stove attached to the chimney for use when the power goes out for a long time in the winter.