I bought a Ryobi 155R cordless 'weed-wacker' several years ago, after I'd used the neighbor's. It worked great....for about a year and a half until I had to replace the battery. Then it worked great again......until I needed to replace the battery again. The battery cost somewhere in the neighborhood of about $50 or more(IIRC).
Instead of a new battery I went out and bought a 2-stroke Stihl for about $230. It's the same as what we used to use at w*rk, and they can take a beating! The ones at w*rk were used about 8 months a year, about 25-30 hours per week. They lasted about 2 years, before needing rebuilding or replacement. At w*rk we also had used Toro, John Deere, and a couple of others, all with varying degrees of success.....Stihl was, by far, the best.
I figure as much as I use one around here (maybe 4 hours a year), it should last the rest of my life!
Also, we've had electric string trimmers...several of them.....pretty much worthless. And a PITA to drag the cord around.
Personally, I'd recommend a 2-stroke model, preferably a name brand (so you can get parts for it, when needed). We had a real nice off-brand that worked GREAT....until we needed to replace a couple of parts. The manufacture was no longer in business, and search the web as I might, I couldn't find or get ANY parts for it anywhere! Bummer!!