You have a few choices to make when you purchase a grass trimmer just as you do when you buy a lawnmower. First you have to decide if you want a corded or cordless electric grass trimmer or a gas-powered one. The pros and cons of each are fairly well-known. The next decision you have to make is whether you want a straight or curved shaft trimmer. The pros and cons of those are not so obvious.
The shaft connects the trimming head and engine. Curved shaft grass trimmers are popular with many homeowners because they are reasonably priced and lightweight, making them easy to carry. It's also easier on your back, which is good for those with back trouble. The down-slope angle of the curved shaft means the cutting head is closer to you. That means you have a lot more control over the cutting process. You'll be able to get into tighter spaces easily. With the extra control, you will be able to get a level cut, instead of the trim ending up looking like a preschooler's bangs after he found the scissors.
While the curved shaft being closer is a good thing when it comes to giving you more control, it's also a con. Since you'll be closer to it, you'll need to take more care to stay safe. Rocks, sticks and other debris can fly up, so wear safety goggles. Make sure you wear jeans or pants made of other durable material to protect your legs. The cutting line will be closer to your feet and toes, so wear good tennis shoes or work boots. No going barefoot or wearing flip flops or sandals. Curved shaft trimmers vibrate a lot, so it's good to wear gloves to keep a good grip and to keep from getting blisters.