will join the bang wagon, and say go for the 12" auger.
and have a second person acting as spotter, while digging holes, putting posts in and tamping. it reduces work about an easy half. (lots and lots of less leg work, and little bit better accuracy. but even still. a larger auger bit, like others have said, gives you that couple extra inches to tamp and line things up. when handling fence posts all day long, and then dealing with shovels. that little extra space is worth it. so much easier to tamp a little more dirt, than it is to shovel out a bunch of dirt.
hydraulic down force kit for 3pt hitch, for me would be extremely helpful for the various dirt / soils on the farm, but *shrugs* i have gotten on without it. with the old regular hand shovel and regular older two handed post hole digger.
having a reversible 3pt hitch post hole digger, when dealing with a bunch of roots. can extremely nice. so ya not having to break out a large pipe wrench, with cheater bar on it. to back out the auger bit when it gets stuck.