you cant go wrong with an 8120. i ran em last year on a crew in wheat, beans, corn, and milo. I was always surprised with what they could handle, as i tend to run em pretty hard, ha. The cabs are great. Very roomy, can comfortably run all day long, day after day
Don't really have anything negative to say or things to watch out for. I really did like the 900 tires, although the duals are a bit better in the mud, better bite. If you're running a 40+ foot header, the heavy lift feederhouse would be nice. we ran 40 footers and they would lift them fine, but with an extra couple hundred pounds it really couldnt.
also, reversing feederhouse and rotor, incab controls of sieve adjustments. they might be standard on the 20 series, not sure, but they sure are nice