If you have small lawn or yard area and there is not much trimming work, an electric trimmer is the best option. However, you need to look at all areas of the yard and examine the plants, trees and grass which will help you to understand how much work you have to do while trimming these areas.
If you think that it will take an hour to trim the yard area, then you should buy a corded electric string trimmer. This is because it does not have a battery and can work for an extended amount of time.
You should also buy a heavy duty extension cord that can help to cover the yard area without any problem. Installing electric outlets outdoors can also make trimming your yard a lot easier.
If you have a smaller yard then use cordless electric string trimmer. It is usually used for yards that do not have thick grass and take just half-an-hour to get the work done.
If you have a large yard and thick grass then buying a gas powered string trimmer is the best option. Buy a four-cycle gas trimmer which is heavy and also works effectively.
Gas trimmers come in curved and straight shafts and it is up to you to buy the one most suitable for your yard. Usually a curved shaft is considered more comfortable for holding.
You should also buy a gas powered trimmer that has a clutch that disengages when the engine become idle. Without a clutch, the trimmer will continue turning around and you have to be careful while standing idle.