I'll add my 2 cents.
I've got a 2004 Bri-Mar I bought used in 2006. It was very lightly used by a landscaper (seriously). Used a little for mulch and more for leaves (with a tall wooden box added on). Mine is a 6 X 10 low pro with 7000 lb. (2 / 3,500 lb) axles. It is a single cylinder with power up / gravity down and has always worked perfectly. I wanted a 10,000 lb trailer, but I just couldn't pass this one up …… since I paid just $2,200 and it really was 'like new'.
I mostly use it to shag my own firewood wherever I find it. I have hauled a little loam to my house with it. I do occasionally sell a cord of firewood here or there by word of mouth. I moved my Kubota tractor with it now and then, back when I still had the tractor.
I made a few mods to it to make it better for me. I added 18" of height to the tailgate. Added 20" of PT boards to the sides and front, and made hinges to make the gate swing to one side. All in all, this trailer has been the second best investment I have ever made. I've done nothing as far as repairs, and (I probably shouldn't admit) I still have the original Chinese Goodyear Marathons on it from 2004. The trailer is just now getting to where it needs some brakes, spring shackles, etc. The trailer has never failed to dump out whatever I have in it. For sure the paint on it isn't great, but probably average for a 2004 dump trailer. It is getting a few rust holes in the left fender from running it on salty roads, but the frame and tub are solid. Within the next year or two I will probably do some type of back yard paint job.
Every now and then I get the itch to replace it with something bigger / better / newer. I've looked at a bunch of different brands that are sold within an hour from home. For me, if I were buying new , I would absolutely consider an all aluminum dump trailer. I'm never hauling stone or gravel, so it would work for me. Money-wise... I would probably end up buying galvanized. I like the N & Ns. No one has mentioned, but several manufacturers have started offering galvanized trailers as an option. Cam and Bri-Mar are two of them. One thing with galvanized trailers.... order it just the way you want it, because you won't want to be welding to it to add this, or change that. You also won't want to drill any holes through it. Also something that hasn't been mentioned is that every manufacturer that I have asked, will de-rate a 12,000, or even 14,000 lb trailer down to 9990 lbs. if you would like them to. This allows heavier axles and bigger brakes/ tires and keeps the trailer CDL exempt. Granted, it leave less legal payload with a heavier trailer, but some may find this beneficial.
In any event, have fun looking at new trailers. Can't wait to see what you buy.