What type of string trimmer line should I use? The easiest answer: probably the heaviest and sharpest gauge that my trimmer can effectively turn without losing power to wind resistance. Assuming your string trimmer has the power, which should be the case for the pro landscaper, you won’t lose cutting efficiency by using a heavier line on lighter duty jobs. And, if you need to jump over to heavier duty trimming work, you won’t have to mess around with switching over to a line that can handle it.
However, for most homeowners, round nylon line will probably tackle the vast majority of simple trimming needs. You might need something heavier for edging and clearing out underbrush, but for handling grass around the house and yard, light gauge round nylon works. Of course, this assumes normal wear and tear on the line won’t bother you too much. If you want to spend less time respooling your cutting head, switching to a more durable material is an option, although a pricier one.