You asked about square versus round...from an engineering standpoint, both will be strong, but from different points of stress. Square tube (or rectangle) your strong point will be on the same axis as the long side of the tube...round tube will give strength around the entire radius (equal strength from all angles) which makes for better material when impact angle is not known (such as in a roll cage versus bumper)...In other words, a bumper will take an impact from a relatively known angle (as in straight on), where a roll cage will take impact or stress from an unknown angle (do you really know how the truck will roll?) so a roll cage from square tube (while easier to build) will have a lesser safety rating than round tube that is triangulated properly (gusseted).
DOM (drawn over mandrel) tubing is the tube of choice (and mandated in most competition) for most high stress applications...it's tubing that is seamless, and will not split when it matters most...build a cage with the worst scenario in mind, and hope you never need it, rather than need it and not have it.
When I build an item for a customer (or me for that matter), I always build with a family in mind...a 4Runner full of kids that flips and the possibility of killing one is always in my mind and safety is paramount.
