I can,t tell you much about the 1680 but I got a 1666 this past fall and can run an 8 row corn head at 3.5MPH and a 25 ft.bean head at 3-3.5 MPH. I might not have the big yields here in central MN , but I'm happy with this combine.
Agree either will work. Personal opinion would go with the 1666. You would have long cleaning system, heavy shaker arms, better hyd., and maybe just about as much power as an older 1680.