I understood that you were talking about an edger that has wheels. I bought a rather expensive one of those that was gas powered and I hated it. I have Bermuda grass in some places which is really tough and I thought a powerful gas powered one would do the trick. Not so...it made a terrible mess because it digs and yet the bermuda grass did not get cut off.
Last year I purchased a Black and Decker string trimmer with a lithium battery. The head turns easily from a trimmer to an edger and surprisingly this thing does the most amazing edge trimming ever. It is really powerful and cuts that bermuda grass right off and also digs a trench in the dirt. I purchased an extra battery because I have a lot of frontage and lots of beds to trim. The battery keeps it's power right up until it's out of juice. The replacement heads, which come wrapped with new string are rather expensive, so I kept the empty spools and restrung them myself.
I just used it for the first time since last year and said to myself that I love this thing. The battery also fits their hedge trimmer, so I bought that too. Really so surprised that the trimmer is so powerful using a battery.