1680 has the large rotor and a larger hopper. Main wear points are the elephant ears and pads, and the transition cone. The early ones with the IH engine are more fuel efficient, but the Cummins in the later ones will out pull the IH engine.
We have both a 1660 and a 1688. Either one will be a good combine. Pending on your farming acres and conditions would help in deciding. We have some hills and the 1660 works better there. (25' head) If I had to sell one I guess I would keep the 1688. You would be happy with either. Easy to mantain combine.
Congrats to you and good luck. I no longer run red due to lack of dealers in the area, but I still think they're probably the best and easiest to set combine.
Good luck with the 1660. I have owned 2 of them for a number of years and have completed different repairs and also upgraded with the AFX rotors. like them for there medium size for my area. Any more questions just ask. Jack