Oh Yeah, if you want to use steel, then it has to be angle iron. Flat steel has no strength when pushed. I also don't know how big the angle iron should be to cover the distances you have from your posts.
Years ago flat steel was used in home construction for angle bracing with fair results. I don't know of anybody who uses it anymore, but I don't know if it's a code issue, or that sheeting is just that much better. The metal straps that were used were fastened to each stud. Again, I don't know how well it worked for homes, but for your project, the posts are way too far apart for flat metal.
Cable on tension would work, but over time, you'd have to tighten the cables as your posts bowed inwards. I wouldn't do this either for the simple reason that your putting stress on your posts from a direction that you don't want to.
Wood is cheap and effective.
Eddie
