square vs. round strength depends on application. square is stronger in bending if the force impacts the flat side of the square.
my 2x2 .120wall rails are still holding up great. they have lots of scrapes but no significant dents. I have like 5 or 6 supports going to them also, so they haven't bent along the length. If you only have 2 (or 3?) supports you should step up the wall-thickness.