If you are planning on a lot of use, go with a hydraulic unit, it will handle rocks better. If you go with a pto driven, make sure it is rated for more than your tractor HP otherwise the rocks could convert it to scrap metal.
I've got a Rotomec PHD-300 and it's great for the odd post hole now and then. If I'm putting in a fenceline though, I go rent a Bobcat with a hydraulic unit. It's a whole lot faster. Also invest in a smaller auger. In my neck of the woods the 9" is fine for corner posts, but the 6" is better for the line posts and it goes down faster.