Aluminum is a much better product, overall than either plastic or steel. Aluminum won’t rust, rot, or breakdown. It is lightweight, flexible, easy to work with. It is safer, with a rounded bead on top and locking stakes.
Important to remember that all aluminum edging is not created equal though. Certain manufacturers make it to thin, and use outdated connection methods.
But most importantly, you said you want it to be inconspicuous. As Andrew stated, Permaloc CleanLine is your best bet for a commercial or high end residential. For most residential jobs, Permaloc ProLine would be more than adequate.