I ran 1680s, 9610s and 9650 STS's for a summer each. This year we're going to run 470 lexions. If you are wondering about different sieve areas and their capacity you also need to consider the stroke length they move and how many strokes they do per minute. That makes a big difference. The NH TR and the CAT lexions don't have shoe augers under their cylinders_rotors. Shoeaugers tend to mix chaff and grain, what you really want to separate. Rotors can be equipted with many different styles of rotor bars in the threshings section and rasp bars or tines in the separating section. This is why people like Gordon, Sunnybrook or St. John Welding selling different setups versus factory. Different rotors are sitting in different angles in the machine. I think the massey is fairly horizontal while all the others climb up towards the back. The veins inside the rotor cage can be adjusted on a CASE. That changes the material flow, threshing and separating abilities. There are people claiming to be able to fine tune a 2388 to run along with an STS. Concaves and separartor grades are made in different styles, too. Go to a big farm show were all the combines are shown side by side and a let a representative take the covers of for you and explain the features. Bring your tape measure, too. You will spend all day and still don't know all the differences in the little details.
