Post hole diggers are pretty basic. The gearboxes are fairly similar and cost about the same. Nothing fancy there from one to the other in my opinion.
The 12 inch auger is a really good size. Anything smaller and you better plan on doing allot of digging by hand to get things lined up. Even with a 12 inch hole, it's almost impossible to get a dozen holes exactly in line. Too many variables that you can't see under the ground to affect the direction of the bit, not to mention that as it goes down, the angle changes right from the beginning.
What you really need to consider, and I did this wrong too, is to get an auger with a good bit on it. The cheap augers have a flat cutting edge that it ground to an edge. This will work fine at first, but will dull in just a few holes depending on your soil. Than you run into the needing more down preasure scenerio when in fact it's a dull cutting edge.
The good bits are either a Pengo or Lowe style. Just a pengo auger will run you around $300, but it's worth it!!! Adding carbid bits will up the price another hundred bucks, and that's worth it too!!!!
If you have allot of holes to drill and your soil is harder than sand, you want a good cutting edge!!!!!!!!!!!!!!!!!!!!!!
Good luck,
Eddie