If you are going to turn the trimming head vertical to use as an edger. The straight shaft will be easier on your back.(no bending over).
But, if you are going to use an edger attachment ON your string trimmer IMO both will work.

I;m tall so I like the straight shaft better. Curved shafts are only rerally comfortable for people a certian height. I remember being 10-11 and having a hard time holding a curved shaft trimmer high enough for it to cut correctly. Straight shaft are also more durable, all high-end commercial trimmers are straights. Get one that will take attachments if you plan on doing other tasks with it, the edger attachment does a better job than the trimmer alone.

If a factor for your work, the straight will also get back under low objects better. Bushes with low branches, the lower rail of a post and rail fence and that sort of thing.

Curved or straight is generally a matter of preference, and generally professional machines are straight shaft, although there are exceptions.

More and more cheap straight shaft are entering the market, because manufacturers know customers believe straight shaft are somehow "professional". On the other hand both Stihl and Tanaka make excellent curved shaft trimmers.

Curved and straight shaft also usually tun in opposite directions. Most engines turn clockwise when viewed from the far end, meaning a curved shaft will cut on the inside (when held on the right), while a straight shaft will cut on the outside. Not necessarily a problem, but one you will have to get used to.

A couple of manufacturers have their curved shaft machines rotate the same at the head as their straight shafts, so their entire family of trimmers are the same. Tanaka does this, and there may be others.