Our dealer orders the M8540s with the same tires as the standard M9540 which I really like. I am sure there are some differences between the two, but they are minor with the exception of the 24F/24R transmission not being available in the M8540. We have no need for a 24F/24R, so this was not a consideration.
We were prepared to buy either the M9540 or M8540. Money was not really a consideration as this will probably be the last tractor of this size we will buy. According to the salesman, the M9540 burns a little more fuel, but if you take out the transmission option, there is little difference beyond the ten additional HP. We were trading in an 80 HP machine which had all the power we needed, so we went with the M8540 and have no regrets. We use ours mostly to pull a Woods DS1260 10'6" rotary cutter and to do dirt work. We have never run out of power. If we were going to pull a 15' bat-wing, we probably would have gone with the M9540.
The engines are the same as are the 12F/12R transmissions.
That is a very good price on the M9540 the way it is set up.