We live in a part of the country where it often gets to twenty or thirty below during the winter and sometimes maintains those temps for two or three weeks straight. We live in a fairly drafty 30'x30' two story plus full basement home with a glass-doored fireplace but no attached garage. The house is drafty because the insulation probably needs a boost and the windows need replacing. We will do both, but it will have to be on a 5- or 10-year time frame as we cannot afford to do it all anytime soon.

We are replacing our 80,000btu 80% efficiency oil burning furnace.

We would like to replace it with a 100,000btu 95% modulating furnace, specifically a York Affinity with the selling point being that it modulates in 1% increments and does not require a special thermostat.

Our question is this...
Since it modulates, does it really matter if the furnace is 80,000 or 100,000btu? We have heard it both ways from furnace people, with some saying that the 80,000btu would be best because it would be more efficient and cheaper to operate. Others say that the 100,000btu model is better because since it modulates, the furnace will only run as much as necessary no matter what the btu's are, and that we will appreciate the higher btu's in the winter cold.

There is only about a $200 price difference between the 80,000btu model and the 100,000btu model.

Any thoughts on which would be a sounder choice?