Trailers are a funny business, we have been selling them for the last 12 years or so, and everyone wants the cheapest they can get. The price is always the key ingredient in every conversation, very few actually look for quality and professionalism. Consequently we are forced to carry the cheapest line we can get. I could not get rid of a high quality trailer, while it is surrounded by cheap ones, everybody wants as big as they can get for the least amount of money.
I have never been like that, I look for materials used and how often, craftsmanship and fit and finish. I have always opted for the better brands, and did not mind waiting and saving till I could afford the better make. So, it was odd for me when I started selling trailers, I had to learn very quickly to stop pointing out the good and start proclaiming the CHEAP price. Most just want to brag about the size of trailer, so they get the biggest they can get for the price they can handle. Never stoping to think about the the construction, materials used or build quality. Believe it or not, there is a huge difference in trailers. It's just not so evident to the novice.
You will ultimately buy the trailer YOU decide on, not what I recommend or what anyone else recommends. I could write a small book of the differences between a cheap-O trailer and a quality built one, but it does no good if you can not afford one. Most people have champagne taste and beer budgets, so talk is relatively cheap compared to actions. Bottom line is trailers are like anything else, they span the range between great and bad, and can be compared to cars, you get what you paid ed for.
I like to look at the materials used, like the main I-beams running the length of the trailer, the floor joist, up-rights and and roof supports. As a mini mun the spacing should be 16" OC for all wall and roof supports, many w use 24"OC for the floor which is OK for the minimum. The size and gauge of materials used should look appropriate, if you look under a 36' trailer and see a 4 or 6" main beam walk away, unfortunately they are out there. The floors should all be 3/4" or bigger,treated or marine grade plywood. Roofs should be one piece aluminum or galvanized, the multi section roofs leak. Wiring should be contained in conduit or run inside channels, the wiring run Thur holes burned in the cross pieces usually ends up getting snagged or shorted by rubbing Thur. Doors should have three (3) hinges to support them not two(2). Hitches should be adjustable for gooseneck trailers and fifth wheelers. The fenders should be Aluminum, not fiberglass or steel. Running gear should be Dexter, and on a car hauler the suspension should be Torsion bar. Also, many CHEAP trailers are built with second quality (2nd's) axles, ask what they use and look ate the spindles, many 2nd's have the grease fitting drilled off center, or bearing races scored. Check the quality of the welds, they are important and should look smooth and even, not splattered and rough. Look under the trailer and check down the main beams, are they already tweaked? Does the materials look heavy enough for the rated loads, are the floor supports correct and strong enough? It is very hard to sit here and tell you what gauge and size everything should be, so I would suggest looking on the web for other trailer manufacturers and comparing what materials they use for similar sized trailers. The better trailers will use Rectangular steel tubing for the floor supports, side walls and roofs. Many better markers will use tubing for the main beams as well. Structurally, the steel tubing is far Superior to angle iron which is the most used material. It also cost much more, so that is one reason the better built trailers cost more, another would be craftsmanship or skill, good welders and fabricators make more money then those with less skill or experience.
If you are going to be loading cars with your new trailer, make sure you get a beaver tail they load much nicer especially for race cars that sit low, they are a must. The side door should be at least 48", they bring more if you decide to sell it. There is so many areas, I cannot list everything. Your budget will dictate most of the options, and on better trailers there is generally 3-4 pages of options to chose from. I have owned many trailers, and it seems every time I went to order a new one, I found new things I wanted on try on the new one, options I hadn't thought of or new things they came out with. On a tag trailer, make sure you get the third member added to the tongue, it really stiffens it up. Good luck and look around and compare as many brands as you can find.