Walk around the yard inspecting the areas that require a weedeater. Make note of obstacles, such as wood or stone fences, metal obstructions or hard-to-reach areas. Identify the distance from the nearest exterior outlet and the overall area of the yard.
At the hardware store, lift and hold several different models to get a feel for the weight. Focus on electrical units if the area that will be edged with the weedeater is small and within 20 to 50 feet of an exterior outlet. Focus on gas-powered trimmers if the area is large or with a substantial number of obstacles.
Read the size of the thread used on each type of weedeater. Minimal obstacles in the yard equate to a limited need for a large thread. Stone, metal or wood obstructions that will damage the plastic line of the weedeater require a thicker thread.
Choose a unit that fits your needs.