Here is my .02 and the standard worth everything you paid for it
20' should be long enough. After length is determined it comes down to about three things:
angle iron/c channel - how big and how long
cross member makeup and spacing (hint 2' spacing saves the manufacturer a lot of moola)
axles/tires
other than paint choice - powder coat/paint and led lights vs. regular the rest of the stuff is marketing. Sure you can argue about ramps and wheel/tires, rub rails, stake pockets, wood floor vs. metal, and the necessity of a manufactured or self made chain bin and winch plate. I have a guy down the road that can weld whatever I want, I just needed a manufacturer to make a solid frame to work from. Everyone customizes their trailers for the most part anyway, so why bother with what the dealer offers, get something plain Jane and pimp it out later. I'd put some spinners on it, maybe jack it way up in the air and put some super fat mudders on it and then run your exhaust through it and coal roll people from your trailer. Yeah Boi.