Hello from South Dakota,
Just my two pennies, but I think it all depends on what it will be used for.
I have used a box truck and it is perfect for housing projects, where you have more than one to do, or if residential work is all you do. They also work well for project vehicles. The biggest draw back is the size, and it is a pain in the rear for service work.
I have also used a utility bed truck and I liked it alot for service work where you do not need much material in the bed. Kind of a hassel to get in and out of the bed and especially if you have a cover on it.
The all round vehicle is of course the standard cargo van. If it is set up with rat packs on the bottom, shelves on the sides, and a ladder rack on top. I have always been amazed at how much material you can get into a van.
So all in all it just depends on what you are doing.
Jon