Maybe you can be a little more specific, are you asking what type of truck as in cargo body, tractor, dump, wrecker? In which case that's a call for you to make, what do you plan on using it for? If you don't plan on pulling around a massive trailer than why would you need a 931? Not that you couldn't build a body of some type to go in place of the 5th wheel plate but you could probably get more use out of a dump(929) or a cargo truck(923).
Are you asking about which generation of the 939 series to get? I ended up getting an 923A2, the newer motor is great and I think has a lot of improvements over the older massive 855. From my short experience thus far and reading different posts here the newer motor gets better milage and lends itself to easier horsepower upgrades if that was a road you wanted to head down. The CTA motor is also very quiet compared to the older motor and you can carry on a conversation inside the cab with a normal tone. Still there is nothing wrong with the older motor in the A1 and A0 trucks as well as it's simple and proven technology or a lack there of. The CTIS in the A2s is garbage and doesn't work 99% of the time, but it's easily removed so tires can be aired up/down and changed with just normal procedures.
Again those are my experiences and opinions and not any hard and fast rules. You will find a die hard following for any of the 5 tons, be it a 939A0-A2 or the older 800 series trucks. Good luck with whatever you decide on.
