Steel edging sounds like a good idea. Steel is a stronger material than composite plastic benderboard that is commonly used, and has a much thinner profile. Instead of 2″ thick edging common with composite edgings, most steel edging is 1/8″ to 1/4″ in thickness. The steel also holds straight lines and curves better.
While these are certainly advantages, steel edging has one main drawback- rust. Steel edging is powdercoated and usually comes in black, green, or brown colors. Once it has been shipped, handled and buried on the project, the powdercoat is usually flaking off. This results in unattractive rusting edge or profile.
Another alternative that solves this problem is aluminum edging. Aluminum isn’t perfect either, it is softer and more easily banged up than steel, but won’t rust. The annonized coloring of the aluminum won’t flake off like the powdercoating on steel edging.