What else will you be using the truck for?
If you use it for towing only, you can probably get by just fine with a short box.
If you ever want to haul "stuff", you may find yourself wishing you had the long box!
After I was done with the fifth wheel, I used my 1994 Dodge CTD 3500 for hauling hay for our horses. I could load 35 small square bales no problem, plus put 100 bales on the flatbed trailer behind it.
Now that the horses are gone, and I don't plan on any heavy hauling, I have the "toy truck" in my sig. It hauls the stuff that won't fit in the PT or the Jeep, and that is all I need a pickup for these days.
Good luck in your search.