About 1/3 of my business is landscape installation and maintenance. 90% of the time when we install some type of edging, we use 4" metal edging. It's 4" tall and about 1/8" thick and is sold in 20' sections. It bends fairly easily, but for sharp bends we heat it with a torch to bend it. It will last about 20 years, and is very easy to 'weedeat' up against it.
The bricks or stones are nice, but their design allows grass to grow between them making it harder to keep neat.
To me, the metal edging looks more natural than the green col-met stuff you get at the big box stores. And its easier to work with, if you have a torch.