Propane-powered trimmers aren't the best choice. You pay more for the convenience of not having to deal with gasoline, and they tend to be heavier.
Gasoline is still the way to go for maximum power. Brands such as Echo and Stihl were tops in the tests for fast, neat trimming and edging. Plus they have the muscle to slice through heavier growth.
The Stihl FS45 is the top-rated trimmer. It did an excellent job edging, trimming, and cutting through tall grass, and the curved shaft makes the $150 trimmer easy to use.
If you want to skip the fumes and pull-starting, your choices have gotten better. There are powerful electric trimmers out there. They start with the push of a button and run as long as you need them, but you are limited to the length of the cord.
But battery-powered trimmers can give you push-button starts and cordless freedom. The Black and Decker NST1024 cut cleanly with one pass and performed adequately in taller growth. It's a Consumer Reports Best Buy at $140. The catch is that the battery gives you only 15 minutes or so of cutting time and takes about 10 hours to recharge.
Whichever string trimmer you choose, Consumer Reports recommends that you wear goggles, long pants, and sturdy shoes. Consumer Reports also recommends hearing protection, particularly with gas trimmers, which can be pretty noisy.