We use reqtangular tubing on rails, and round tubing for roll cages of the race cars. The left sides of the car also use a heavier gauge to keep them on the track better. The reason for tubing, less flex, we make all track adjustments with suspension, and, this method should be taken to day to day routines with trailers/and vehicles.
Build your frames rigid, spand a few hundred dollars more on steel, and do the job once. When you want a betetr ride, or better trailering, make those adjustments with suspension. Peronally, for trailers, I prefer spring suspensions over rubber toque bar. The torque bars wear faster, and get sloppy eating rubber from the tires, where the springs ride rougher, but last longer...
When you wire your trailer, use 6 strand, 12 gauge encased semi truck trailer wire. Then use loop ends into a junction box out to lights. That way you don't have to worry about wiring issues down the road... s