Opinions are like.... You know. Everybody has one. As far as round vs square, I would prefer round. It is easier to get threaded bungs for it then. In My Opinion, welded seam tubing is fine. I built my short arms with (insert climactic music) schedule 80 pipe. Some people on here will say it is an accident waiting to happen, and how pipe is for poop and yada yada yada. They are strong, they are working great. It is up to you. Round DOM is going to be the strongest, and easiest IMO, but it's not cheap either.