The single stage furnace is basically a car that can only go on one speed and that's it, nothing more or less it gives you the same result over and over. Single stage furnaces only have one heat output which is high, so no matter what the temperature is inside your house if it needs to get hot due to the fact that it is colder outside the furnace will heat up the house. Also the single stage furnace is also louder than all other furnaces because its always running on full. The two stage furnace has dual options, where it heats up your house only using around 40 - 60% of the energy from the furnace. but once it picks up that its even colder outside and the temperature still needs to get warmer it will kick in to stage two which is full speed running at full efficiency. The two stage furnace is better because it only runs on full energy when it is needed so most the time you will be running on the first stage which is not full energy being used, which also means you are saving money. Unlike the one stage furnace which is running at full energy the whole time which is more costly.These are the differences between a single stage furnace and a two stage furnace, in my personal opinion i would suggest going for two stage because it will help you save more money in the long run, and it is also much quieter than the single stage.