We run 12 speeds in a 9200 and 9400 and we have never felt lacking for speed selection but we just gear up and throttle back if we need a different "gear". The bigger engines put out as much horsepower at 1800 rpm as at rated speed so you are developing the horsepower at the lower engine speed anyway and using less fuel to boot. Not sure if that would be true of the 9100 or not but it is just as heavy tractor as the bigger ones with a smaller engine so it would suffer the worst of any for lack of speed selection. I sure would not be afraid of the tractor with a 12 speed but there might be times you wish there was an in between gear with that HP tractor. Worse case is you would end up going a half mile slower than what you might be able to if you had the 24 - only you can know if that is an aceptable compromise. One thing about a 12 speed - less to go wrong as the hours accumulate. If should be no worse than a Versatile or other similar tractor with a 12 speed manual and they have farmed a zillion acres. The 24 speed is actually an overdrive unit of the 12 speed. The 24 will have a higher top road gear because of this also.

John