IMO....curved or straight depends on what you will be using it for. Curved is much better for detailed, around the home work. Straight shaft if you are clearing vast areas of grass/ weeds and not concerned as much with detailed work.

Aren't Hitachi's the ones Home Depot rents out??

1. Will you be able to stand up straight and use
   the curved shaft? If you have to work bending your back, I feel it
   will be a pain.
2. If it’s a commercial straight shaft, that could
   mean a solid drive shaft and extra weight vs a cable.