We recently switched from a 9N to an 850
(850 is just like 650 w/bigger engine/driveline).
Why? - Didn't like working on 9N, 850 is easier to work on. - 850 better, brakes, axle seals, engine, etc. - Wanted more power (650 somewhat more than 8N). - Live hydraulics is nice (but, the cost is that it probably isn't as reliable as 8N).
8Ns DO have parts availability going for them, and
a lot of them have the Hi/Lo Sherman gear boxes.
If I had to do it again, I would hold-out for a 660/860 w/Live PTO,
it opens up a whole new world of what you can
do with your tractor.
...as mentioned on other response, cool things
like snow-blowers become practical w/Live PTO.
We have not had much success pushing snow with
a 2WD tractor, it gets stuck surpisingly easily.