You can't beat a true edger for ability to quickly get the job done and
to cut through areas that haven't been done in a long time. The
tradeoff is they cost more and are one more thing to store.

I have a Troy Bilt combo string trimmer and edger. I'm very happy
with it. I have about 150 ft of edging to do and it works fine as long
as you do it about every month or so. If there is a lot to do and it
hasn't been done in a long time and is tough, then it may be best to
rent a regular one for the first time. But given that it changes from
an edger to a trimmer in a minute and stores easily, it's an excellent
choice for the right application.