DOes the 80K unit you have now keep the place warm enough?
It may be too big. There is a bottom limit on how low the modulating furance can go, so on a mild day, the smaller one could end up being more efficient since it could run longer at lower firing. You get the most efficiency and comfort if the thing runs all the time rather than turning on and off, so modulation really can help. Another option that I really like is a variable speed fan. These ramp up and down with the need and there is no rush of cold air when the furnace first turns on.
Have you done a heat load analysis? That should really be the first thing to do.
Keep in mind that an undersized furnace doesn't mean the house will get colder and colder at the same outside temperature. If the furnace was sized to maintain 70-degrees at an outside temp of -20, it won't just fail if it gets to -21, the inside would drop to 69 and stay there. Depending on how much insulation you have and since the coldest temps are usually just before dawn, that would mean it would drop slowly overnight, and may never actually drop below that 70-degree mark.