One's a single stage burner, the other is a two-stage burner. The most comfortable heat is one that runs all the time. Since the winters aren't always super cold, for those mild days, the first stage is often enough to provide all the heat you need. Also, it's more efficient - same sized heat exchanger, but less heat going in means that it can extract more. The difference isn't all that much, but it's there...the bigger thing is the comfort level.