the 6x12 needs 5 1/4" drop
and the 5x10 needs 6" drop
to have them leve and loaded with 300 pounds of thong
i have 4" as well.
i prefer the 6x12 uhaul trailer that is more stable
i have towed 4k pounds for 600 with no issues.
dual axle trailer drive better. they distribute weight better and have less sway and seem to me to have less drag. i feel my car work harder pulling single axle than a dual axle.
and recently towed a mini cooper on a car dolly for about 100 miles. total weight was close to 5k pounds, kk towed like a champ
don't have the pic with the mini on the trailer