There are two types of post hole diggers. The good ones are hydraulic. They cost a lot more, but they have reverse and you can mount them to your front end loader, or even better, to a backhoe or excavator if you have one. They 3 point only go in one direction, so if you get it stuck, you have to use a really big pipe wrench to get it out, or a backhoe to pull it out. I've done both, the backhoe option is faster, but your get a holes twice as big as you wanted.
Then there are the cutting teeth. The good ones are replaceable and made of carbide or some other hardened steel. The cheap ones have metal cutting edges or come with replaceable metal teeth.
In my opinion, all of the 3 point models are about equal as far as gearboxes go. I have a low budget Lienbach Line and it's gear box is indestructible. No reason to spend more money there. What you need to spend the money on is the teeth. I've found the best deal is at Tractor Supply with their Country Line models. You don't need any down preassure because it just digs through the very hardest clay possible, during the driest months of August. The only drawback to it is it digs too fast and if you lose control of how fast it's digging, you will bury it and need to either wrench it out, or get a backhoe to pull it out. I've busted pretty big pipe wrenches with even bigger cheater bars on them unwinding an auger out of the ground.
If you are digging in rock, then hydraulic is the only option, and you need special teeth for that.
