You proposed a very interesting question, and I have enjoyed reading the answers, as I also build quite a few things here around the farm. Sometimes these projects are necessity, and sometimes for the pure enjoyment of using something that "I" built.

Now, since you further expanded what you are wanting to do, or purpose of the project, most rear hitches I've ever built have used rectangular tube, and even for a front hitch, thinking of a trailer project I just finished. Standing the vertical tube up, to get the most vertical loading out of it, but also in this situation, a channel iron gusset/brace, will reinforce the horizontal, or side to side loading. To use strictly a rectangular tube, or even a square tube, what you would need to decide, is which way the most force will be exerted on the tubing. If you have equal forces, both directions, then it's simply figure the strength needed, and a square box tube. If the forces are unequal, then using a rectangular tube becomes more prevalent, with focusing it's direction of use based on the forces applied. Simple.......... no, cause with farm equipment, we sometimes get that third element, and that is what I'll just call twist or maybe better stated as rotation of the plains. With what we put it thru, the force we apply, doesn't always stay in a vertical/horizontal plain, to the actual structure itself........ if that makes sense??????