I'm not familiar with the term "dry van" but from your description, it sounds like an enclosed trailer hauled by semi trucks. If so, then either should work just fine depending on the price and delivery. The problem with the trailer is the axles. We used to have several for storage when I worked for Airborne Express. They were water tight, and solid, but the tires would always go flat after sitting for so long. We needed them at dock height to run the forklifts in and out of them, so it was an added expense to fix the tires when they went bad.
If I was to buy a trailer, I'd want to take the axles and landing gear off of it and put it on the ground. The roll up doors at the end tend to get pretty beat up, but they do a fine job. Probably not a secure as double swing doors, which are also available. The trailer will be taller and usually have a side door on them, but not always. None of the ones I was ever in had fiberglass roofs. They were all metal to the best of my memory.
The sea container is going to be more solid and ready to go. You might be able to drive a compact tractor into a trailer, but with a sea container, you will have to fold the ROPS for sure if you want to put it in there. I had a 40 ft container that I stored two four wheelers and my 35 hp tractor in it, plus shelving for tools and enough room for dozens and dozens of boxes.
I put down a bed of gravel for mine. Animals liked to make a home under it, so I went around filing in the gaps with concrete. That helped, but it wasn't 100%. My neighbor now owns them and he says that he has a great big rat snake living under one of them. I had two. He likes it because he says it eats mice. I'm all for killing mice and like to take care of them with poison. He prefers the snake method. To each his own. LOL
My brother has a 40 ft container behind my house right now. He has it full of all his personal stuff while he looks for a house and land to buy in the area. It's just sitting on the dirt and has been for the last two years. We're not sure what it will take to move it when he finds a place. I might even buy it off of him. We'll have to wait and see what he finds and what it will cost to move it there. I didn't have any moisture problems with mien, but I was in them every day. His sat for months at a time and he had allot of issues with moisture in his. We added two vents to either end of it and a big turbine to the roof. With air flow going through it now, the moisture problems seems to be minimal to non existent now. He's also using it as a wall for his dog kennels on one side. He just screwed his panels into it and created four nice kennels ten feet wide there. On the other side, he's raising ducks. It's a great wall to build off of and for the price, an excellent storage building.
With a storage container, you pretty much get the same thing. There isn't allot of variation from one to the other. Some are just more dinged up then others. With the trailer, you have allot of options such as length, doors, roof and materials used in building it. You have axles and landing gear to deal with too. I looked at one trailer for sale and it was total junk. I wouldn't want it for free, so I bought the containers. My brother did the same thing, he couldn't find a trailer that was any good, but had allot of containers to choose from.
Good luck,
Eddie
