If you have a large or rural lot to maintain, the more accommodating straight-shaft model is the best choice. Safety Concerns Eye protection is a must, and ear protection is highly recommended. Gloves and heavy shoes are a good idea. Never install a hard blade without the proper hardware. Never use a string trimmer to cut grass around the base of a tree, especially thin-barked young trees, which can be damaged easily.
Some trimmers are more user friendly than others.
For instance, a translucent fuel tank makes it easy to know when to refill, and then do it without spilling. On some engines the air cleaners or spark arrestors are buried or difficult to reach for maintenance. It's hard to beat the national hardware and home center chains for bargain-priced trimmers and brush cutters. However, if you buy from such a place, be prepared to assemble the unit and diagnose for yourself any difficulties that arise. In terms of value, a local servicing dealer may have the edge.