By what can be read on here. Most units are the same. Just the quality of install, if I've read any of the posts right. Obviously a 2 stage is better than a single stage. And a 90% is better than an 80% furnace. But apples to apples, they are the same to a degree, most utilize the same parts and are interchangeable to a point. And oh yeah, it is fun to argue. At least you can fight back with some intelligence.