2-cycle engines require that you mix fuel and oil in a gas can, then add the mixture to the tank. Also known as 2-stroke engines, this type of engine is lighter and simpler to maintain than a 4-cycle engine.
2-cycle engines also have the potential to pack a lot of power into the same space as a 4-cycle model. This combination of being light-weight and having plenty of power gives 2-cycle engines a great power-to-weight ratio.
4-cycle handheld string trimmer engines are designed more like a car's engine. Gas goes in the fuel tank and oil goes into the crankcase. They're more fuel efficient than 2-cycle engines as well, so you get more use out of every tankful of gas when using your 4-cycle string trimmer.
A 4-cycle engine also has the capability to produce more power overall than a 2-cycle. This typically results in a larger engine in terms of size and weight, which can have drawbacks for those who are physically unable to sustain the trimmer for extended periods of time.
Shaft Style Curved-Shaft String Trimmers are easy to maneuver for flower bed trimming and more simple tasks. Because the head of the trimmer is curved inward, you do not have as much flexibility to trim in hard-to-reach corners and crevices.
Straight-Shaft String Trimmers are the tool of choice for landscaping pros because they have plenty of power and torque to rip through thick grass and brush. This is the tool for trimming under fences and other tight places.