We each have our personal preferences and your mileage may vary but here are some of my thoughts regarding which truck might be best.
The M52 and M818 have single circuit air assisted hydraulic brakes. If you lose one line you lose all the brakes. The transmission doesn't give you any braking advantage other than friction.
The M931 has spring air brakes and at least if you lose all the air the brakes will lock up. The Allison auto trans is set up to slow the truck through transmission braking if your foot is fully off the throttle so you have another advantage in braking that the older trucks don't have.
Lastly, if you ever want to put a wet kit on the tractor I believe all the M939 series Allisons have a toothed torque convertor and two pto ports just like the M915A1s do. If my memory is correct you will get about 1/3 the rated HP of the engine at 1,000 RPM so you can run a pretty stout hydraulic pump. The M932 has a hydraulically driven winch so it may be a better candidate for a wet kit.
Another option that is close to the same general price range is the M915A1. 400 HP, great brakes and a lot more towing/hauling capacity. You'd lose the 6x6 though. The M915A1s all have toothed flywheels and two PTO ports. They also have Jacobs engine brakes set up for the driver to select 2, 4 or 6 cylinders of engine braking. That can be a big advantage when in hills or running heavy.
The M931 and M915A1 also have brakes designed for modern loads and highway speeds. The M52 and M818 were designed for a 45 MPH max road speed. All the M939 series trucks should have been modified to have ABS brakes. A few leaked out for sale without them though. The ABS is another safety advantage if you ever play on slippery roads.
The key to driving the Allison with a load, whether in the M939 series or M915A1 trucks, is to use the gear range selector to downshift early when the RPMs start to drop on a climb to keep the RPMs in the high end of the range. If you let the engine bog down to the bottom of the RPM range (when the transmission is usually going to downshift) you'll be hard pressed to get the RPMs back since you'll already be near the bottom of the power band or already below the power band after the the shift.
If you select the lower gear when the engine is turning at a higher RPM than the transmission can safely downshift at the tranny will not downshift right away but will do so as soon as it can do so safely. That results in the downshift happening soon enough for you to wind up at the very top of the power band and allows you to climb at a much higher road speed. If the hill looks like a 3rd gear hill downshift right to 3rd and let the tranny do it's thing to keep you at your best road speed. On the up shift you will likely want to go one gear at a time as the truck reaches the top of the power band. That way it won't shift right back to 5th and leave you lugging.
All in all, for you application I would not consider any of the older trucks with single circuit brakes and standard transmission as the Allison and spring brakes really do make for great driving experience.
I hope some of this rambling helps.
Lance
