You need to determine how the digger will be used. If used primarily for putting in posts (4-6") then I would stay with the 9" auger. Large holes require much more dirt to be tamped in around a smaller post. Tamping in the dirt to get a solid post takes the most time and work. It is also harder to get a 12" auger to go in the soil in my area (lots of clay). If you want to plant a few trees, the 9" can also be used to dig 3-4 holes close together and the rest dug out by hand. I have 6, 9 and 24" augers and the 9" gets used the most. 6" rarely used and the 24" for trees.