OK, I've run both series heads in beans in both great, and not-so-great conditions. The III's allow a little faster ground speed. This is because of deeper skid plates and a more flexible cutter bar. It can be adjusted more finely for a closer cut. This is because of the eye bolts on the end of the arms that can be run in or out to adjust the guard points pitch and the draw bolt on the back of the arms that adjust the tilt. I also think they feed short beans a little better, since the transition from the cutter bar to the pan is quite a bit smoother. The reel motor and hyd. pump is ok, but I think it's real usefulness was if the combine was equipped with an auto reel speed module that increased or decreased reel speed with the ground speed of the combine. Other than that, the only uses I've had for it is to have a hydraulic flow source I could tap into to drive my chaff spreader. The auto header height function is a little more refined, but works on the same principle.
When it came out, Gleaner heads equipped with the Hart-Carter floating bars were way better bean heads than those fielded by other manufacturers, and I think they still are good so long as you recognize they're limitations and operate accordingly. Match your ground speed to field conditions and it will do an acceptable job. Any other questions feel free to ask!