The most important aspect of a utility trailer's construction is the frame, and there are three shapes of metal that are used to build a utility trailer's frame. What shape is used ultimately impacts how much a utility trailer costs, how much weight it can support, and how durable it will be.
Angle Iron: Inexpensive and Suitable for Basic Jobs
While all of these shapes have angles in them, angle iron specifically refers to metal supports that have one angle, a 90-degree angle, in them. This 90-degree angle makes it easy to attach two pieces (e.g. the utility trailer's box and other pieces of the frame) together, while providing some support. Since pieces of angle iron have only two sides, they're an inexpensive material to construct a utility trailer frame out of. They also don't support as much weight as pieces made from other shapes, though. If you only need a small utility trailer and don't haul heavy materials, a trailer made with an angle-iron frame will be your most affordable option. Just make sure the loads you haul don't exceed the trailer's maximum weight, because the weight limit is likely to be fairly low.
Channel Iron: Substantially More Support
Compared to angle iron, channel iron adds one more side to the metal pieces of the frame. This shape has three pieces that connect at two right angles, creating a channel or "U" shape. The shape requires a little more material, but it adds a substantial amount of additional support. You can expect utility trailers with channel iron to cost a little bit more than those made from angle iron frames, but the increased sturdiness of channel iron greatly improves how much a trailer can hold and how long it will last. For something that can handle most hauling jobs and will serve you well for many years, channel iron is an excellent choice.
Box Iron: Long-Term Durability
Box iron adds one more side to the frame's pieces, effectively topping off the open channel of a channel frame so that it has a four-sided box shape. This adds more rigidity, and it especially increases the long-term durability because every side has a parallel side. Should one side start to rust years from now, the opposite and parallel side will still lend support.
Utility trailers with box iron frames will cost more because more metal is needed to build the frame's pieces. For something that'll last as long as possible, though, this one-time investment provides excellent value.