I don't know about your tractor and post hole diggers, or the gear box on the Countryline post hole digger, but I can comment on the auger itself. I have a 37 hp tractor and I bought a Leinbach Line post hole digger with a terrible auger. I replaced it with a Speeco auger that was a lot better, but the replaceable cutting teeth would wear out quickly. The store I bought it from had a few in stock, but when I bought all of them, they never got any more. So in desperation, I went to Tractor Supply and bought a Countryline 12 inch auger.
The Countryline auger was so much better at digging then what I had before that I cannot even compare how good it is. It went through my summer baked hard red clay like butter. Then when it hit what I call the concrete grey layer of clay, it went right through that too!!! In fact, it drills so quickly that I have to make sure to slow it down because it will bury itself and I don't have enough power to turn it once it's all the way in the ground. I have a 24 inch pipe wrench that I put a 8 foot pipe onto for more leverage that will unscrew it, or I can just use my backhoe and pull it out of the ground and make the hole twice as big as it comes up. I've done this a dozen times now, and it's always something that I promise myself that I will never do again. But then I do it again.
I got the crazy idea to buy a 6 inch auger for some smaller holes so I wouldn't have to fill as much material around them. That went in even faster and so deep that it bent my driveshaft. That might be the biggest mistake I ever made in drilling holes!!!
If I was buying a 3 point post hole pole hole digger today, I would take the chance on the Countryline gear box and auger. I would not want an auger less then 12 inches. I've done enough fencing to know how hard it is to keep 3 posts in perfect alignment. More fence posts are even harder. It doesn't matter how perfectly you start the hole, it moves as it's going down. Roots, rocks and softer dirt will all change where the hole is and you will have to use a clam shell digger to manually shave off one side of the hole to get it perfect. A smaller auger will mean more digging by hand to line up your posts.