There are many different string trimmers on the market today. For those that don't know what this product is, here is a brief description to help you. A cordless string trimmer is a device with a long handle and a motor on one end. There is a "string" on the bottom side, which rotates at high speeds in order to cut the grass along the edge of driveways, exterior walls, and for clearing light brush. While some models use a metal cutting blade, string trimmers use a nylon string, usually one that has some kind of automatic feeding device to cut tall grass.
String trimmers come in two different models. One is an electric trimmer, then you have gas motor trimmers. Each of these has pros and cons, and this can be a key component for which model you buy. If you want something that can do light jobs, then an electric model can be a good idea. They are lightweight and easy to handle, and can be plugged in for convenience. With an electric trimmer you only have to stop to replace the nylon string on occasion. They can be cheaper to buy than their gas powered counterparts, but there are some drawbacks. Typically, the electric motor isn't as powerful as a gas motor. They are all right for edging, but if you have a large space with tall grass that needs to be trimmed, then using these are not a good idea. Because they are electric, you have to have them plugged into some outside outlet, and you don't have quite as much range.
Gas powered grass trimmers are powerful, but lightweight as well, and they usually have a two stroke gas motor at the end. These run at a much higher RPM, so they can cut through thick areas of grass overgrowth quickly and easily. The also have steel blade accessories, for cutting down small saplings, and bushes. Because they are gas powered, you can go just about anywhere the job takes you, and are not limited by cords. There are some draw backs with using gas power trimmers as well. For one they are typically more expensive than electric. While you can take them wherever you have a job, they have small gas tanks, so the larger and more difficult the job, the more often you have to stop and fill the tank. Because these are two stroke motors, they have to have a special fuel/oil mixture, and even already mixed it can be a hassle to fill the tank. You also have to consider that because of the high speed, you may have to change out the filament more often as well.
In the long run, buying a cordless string trimmer really depends on your needs and budget. You can find many of the electric trimmers for just around $100, and gas trimmers start at around $125. If you are just trimming the edges of your lawn, and don't have a lot to deal with, then you should buy an electric model. However, if you have a large area to trim, high grass, or are using a trimmer for some kind of lawn care business, then you are better off buying a gas trimmer, they will last longer, and perform better.