I am currently running R-62's and I have been well pleased with overall performance. I would think that it would be hard to get away from the gleaner system as far as the accelerator rolls, which definately give the gleaner an advantage over every combine out there. Plus the overall simplicity of the combine and being able to walk around the engine for routine maintenance. I believe the new R-65-75's come with 2yr-2000hr. warranty on the machines. Along with the numerous improvements larger rear feed shaft, larger clean grain elevator, S-39 final drives, hydro driven from engine resulting in ultra quiet comfor tech II cab. New can bus monitoring that can show the exact rpms of important areas of machine w_nice diagram of combine. Plus the new 5 series come standard with 14" diameter unloading auger, along with chrome hardened concaves,cage,helicals,augers(w_1_4" thickness in high wear areas) throughout the machine. As for the challenger 670 I have no personal experience in running one. Although I have heard good things about them. I have only seen the 670 and the massey 9790 at farm shows. The combines have alot of nice features from the hyd. drive rotor (no right angle drives on machine) to only having 31 grease zerks and 9 belts and 4 chains. I do like how the beater is incorporated into the rotor. I believe the challenger_massey has the gleaner 4-spd hydro- traction drive, high cap. cleaning fan, feeder house(40"gleaner,44"challenger660_massey9690,55"challenger670_massey9790 these are widths of feederhouses) and they also have same rear axle, and final drives. Both the gleaner and challenger,massey use the same headers with feederhouse width being main difference. Overall I believe both machines would do an excellent job. I would also be curious as to how an R-75 with standard options straw sprdr. no chopper,no rwa. 30' 7000 series hdr. std. 30.5-32 compares in price to 670_9790 with similar options. Hope this info. helps in your decision.