I bought my first weed-trimmer used, a cordless model by Troy-Bilt if I remember correctly, from the same couple I bought my house from. I don't remember what I paid for it, but whatever it was, it was too much. The thing was heavy and had a battery life that could be measured in sneezes--not nearly enough to get through the weed-trimming chores on even my small urban lot. When the battery ultimately gave up the ghost and could no longer be recharged, and I discovered that the cost of a new battery was roughly the same, or in some cases more than, a completely new weed-trimmer, I moved on to another models, first cordless and then finally corded, each one with its own set of disappointments and frustrations. (I've never tried a gas trimmer, however, so my comments here will not address their pros and cons.) Along the way, however, I learned a few things I wish I'd known from the get-go.
Size Matters--As Do Shape and WeightHaving purchased a couple of weed-trimmers online, I learned the hard way that it's a good idea to actually go to a store where you can see and preferably hold the tool to get a better sense of the size. There is typically an adjustable shaft that you can set to a length that will hopefully work for you, but if you're rather short or particularly tall, some models will have you either stooping to reach the weeds if the trimmer's too short for you or holding your arms at an odd angle if the trimmer's too long. Taller people tend to prefer models with a straight shaft; shorter people generally do better with a curved-shaft trimmer. You should also consider the weight. Because of the angle you need to hold a weed-trimmer at, even the lightest models can start to feel heavy sooner than you might expect. Make sure you're going to be able to handle it for the length of time you'll need to get through your clean-up.
Feeding Matters TooCheck out how the string feeds out from the spool. Some feed automatically, others you have to bump against the ground to feed out more string. That bumping thing gets old real fast, so look for something that feeds automatically. I also found that I prefer dual-feed trimmers--that is, ones that feed out two lines from one spool simultaneously. They just seem to cut more efficiently.
Cordless Is Not Necessarily as Convenient as You Might ThinkWhen I first started using a weed-trimmer I assumed that the cordless models would just naturally be more convenient than dragging a long electrical cord around behind me. But the weight difference between cordless, which can run upwards of about 8 lbs., and corded, which generally weigh between 5 and 7 lbs., can easily cancel out the benefits of the cordless models' go-anywhere range. Those extra few pounds can matter greatly when you have a lot of trimming to do and you have to take breaks because your arms and back are getting tired. Ditto when you have to take breaks to recharge the battery on the cordless model--especially since the breaks will need to be several hours or even a day long for a full recharge. I found that with a 50- or 100-ft. long heavy-duty extension cord, a corded trimmer was more convenient for me because it was lighter-weight and could run as long as I needed, making my most-hated garden chore quicker to get through.
Price Does Not Always Reflect QualityThis was the biggest surprise for me. When a heavier-duty, higher-priced cordless model I'd been using and cussing at on a regular basis finally bit the dust in the middle of cleaning up my driveway, I got mad and stormed over to Home Depot, where I purchased the least expensive model they had--a corded 4-amp trimmer by Homelite. It is smaller, and therefore more comfortable for someone of my height (5'4"). It weighs in at about 5 lbs., so I can generally get through what I need to do without inciting a backache. It has an automatic, dual-feed line that rarely sticks like some of my other trimmers did. All in all, it has been more dependable and easier to use than any other model I've tried. The cost: just $30!
(It should go without saying that it's important to follow all the manufacturer's safety recommendations when using any string-trimmer, not the least of which is to wear fully closed shoes and safety glasses.)
I still hate using a weed-trimmer but finding a model with fewer annoying issues helps me hate it a bit less. There are certainly other considerations to investigate when choosing a weed-trimmer but these are the issues I particularly wish I'd known about up front. I could have saved myself a lot of swearing over the years.