I have a six inch and a 12 inch auger. The six inch is useless. Total waste of money. I use the 12 inch for everything. If I want to put a one inch pipe in the ground, I'm using the 12 inch auger. I'm not changing it for different sized post. I use it for 4x4 square posts, 6x6 square posts and 6 to 8 inch round posts. Most of the time it's fine, but if I need to install a bunch of posts in a straight line, every 8 feet, I'm going to need my clam shell digger to modify half of the holes I drill. The auger will either angle off to one side while drilling, or hit something a foot down and slide the entire hole over.
In the last 20 years of installing fence posts, and moving them to other locations when I change my mind, I've learned that the most solid fence post is the one set in concrete. It takes everything my full sized Loader/Backhoe has to pull them out of the ground!!!! They don't want to come up, not for nothing. And the posts that I set in the ground with just dirt packed around them all pull out easier then T-Posts. I don't know of anything that pulls out easier then a post that's had dirt packed in around it.