In most cases, the Straight Shaft weed eater is favored over a Curved Shaft weed eater. The reason is "better reach."  You can easily reach under bushes with a straight shaft trimmer, which is more difficult with a curved shaft.
That said, A Curved shaft is a more comfortable tool to use if you have back problems or find a gas weed eater heavy to use. Because of the curved rod, you don't have to bend over to use it, offering more relief on the back and arms.