It may have to be slightly larger than the square stock in order to retain the same rigidity, but it will be stronger maintaining the same wall thickness as the square material. The round cross sectional area will handle both torsional and axial loads much better than square or rectangular tubing will. This is because of the way the forces or stress is displaced around the entire circumference of the round area, wheras the stresses placed on the square stock will be amplified in the corners and in the seams. It's the same reason you never see a square hole in a car frame, the corners give stress risers and fractures a place to occur. By design round has equal strength in all directions. A circular shape simply spreads the load more evenly than the square or rectangular shape. The weakest part about square tubing is the corners. A round tube is a continuous corner and therefore very strong. A round tube may not be as stiff as a peice of square stock of the same dimension but that is eaisly fixed by increasing its diameter or wall thickness. Now, would you care to eloborate as to why you think square tubing is stronger the round tubing? There just rock sliders...it's not going to make that much difference either way. Just use which ever one you like best. I personally just don't like using square tubing on my rigs. If you don't have access to a bender and notcher, the square material is proably going to be easier to work with. If you use square I would recommed atleast a 3/16" wall thickness...I would use 1/4" but I like over kill...
