Which is best for pulling a fifth wheel, short or long box?

For pulling, the long box without question. Short box works, but is a compromise with non-towing uses.

More importantly, for a 37' fiver, you will want to consider a dually, and as far as I know, they are currently only offered with a long box. When you do the math on the load capacity you need, the box length decision may be made for you.