I am often asked "What Edger do you recommend".
Having been designing and installing landscapes since the mid 80's i have some strong opinions about edger. This comes from having installed 10,000's of lineal feet of edging, and supervising the installation of 10x that.
Metal edging in what I most often recommend for a few reasons.
First, it is important to understand how far metal edging has come in the last 5 years. We all remember that razor sharp edging and the associated black safety cap that was a pain to install and did not stay on for long anyway. That edger has been replaced by Rolled Top Edger where the top 1/4 in has been mechanically folded to create a rounded edge. Now the only danger areas are then ends if not installed. This type of edging is available in several gauges (Thickness). An upgrade to Rolled Top Edging is "ProSteel" this edger has interlocking tabs that the edger pins recessed into. The edges are rounded and the edger is painted green. This edger has been used on commercial projects for 15+ years.
Many people like the look of concrete edger. I have some serious reservations with this type. Being a very picky designer type, I am always adjusting and readjusting the metal edger to get the perfect curve or an absolutely straight line. With concrete you just do not get that perfection. You are working with a material that sets up fast and a machine that is not exactly high performance in the steering department. There is no going back once it is on the ground, if you want to change a bed you are out of luck especially if you went with stamped and colored. It is nearly impossible to match an existing edging. My next issue is that the concrete is placed on top of the grade and typically is not recessed into the ground like other edgers, allowing for aggressive roots to penetrate underneath the concrete. In Colorado we have the issue of expansive soils. During the wet seasons the ground swells and in the drought years the soil shrinks causing concrete to crack and or have voids under the edger. Water retention had also proven to be a big issue. If great care is not taken to pay attention to drainage patterns the flow of water may be stopped, especially bad if the edger hold water on top of plants or near a foundation. If this occurs then cuts will need to be made through the concrete- ugly.
Bender board Type Edging, I have seen and it is OK. the main problem I have noticed with this type is the Freeze thaw cycle in Colorado will have an hydraulic action and can lift the edger up and out of place. Another problem is there are not pins or stakes that are made specifically for this edger, so you must improvise. Most often wooden staked are pounded into the ground and screws used to secure the edger. My last hit to this edger is the damage caused my an accidental mower blade or over zealous string trimmer.
Plastic edger has many of the same issues as the bender board but does not hold up to Colorado's high UV light during the hot summers.
I have to say that no edger is the silver bullet to end all of our edging needs, as soon as i figure out the magic combination of ease of installation, longevity, and flexibility I will be calling from the mountain tops.