The 9n is the older model, and it was built around 1939. The 9n tractors have a three-speed transmission, and it is less potent than the 8n tractors. 9n's have the left brake pedal and clutch on the left. The 9n also has footpegs.
The 8n started production eight years after the 9n was built in 1947.
The 8n has a slightly higher horsepower than the 9n. The 8n tractors have a four-speed transmission and a much better steering box. 8N's have both brake pedals on the right. The 8n also has running boards instead of the footpegs on the 9n tractor.