The 9000 movements have a smoother crown operation, when changing the time/day. The tolerances when adjusting the hands are tight and it feels smooth clicking over the date.
When the date switches over on the 9000, it's practically at midnight, whereas the 8204 switches over not as close to midnight.
When adjusting the time/date/day on the 8204, you feel the gears, the clicks are rougher and tolerances feel and look looser when moving the hands around. Clicking over the day/date feels clicky. However, the 8203 movement is Italian Marine Spec, so I just assume the roughness is due to robust military grade build and consider it character. I'm assuming the 8204 is of the same military-inspired lineage.
To me, the biggest differences are in the tactile feel and character, when changing the time/date. I don't particularly notice a difference in accuracy between the 9000 and 8000 movements.