I ama 9N fan. They are simple machines and easy to keep running in the field.
The 8N is more popular and parts are easy to find. It also comes standard at 12 volts unlike the 9N which is six.
Most 9'stoday are converted to12. Mine is.
I believe the engine of the 9N is slightly larger. 20HP vrs18. But it is low compression.
Operationally, I cannot see a difference except for the live PTO on the 8N disengages with the clutch. When you stop with the Brush hog engaged, the spinning blades will push you forward. There is a coupling available that prevents this, but it is a dangerous feature.
I am a fan of the 9N but frankly, you would be happier with the 8N as it is "technically" more modern. Both are decent machines for the purposes you have intended.