I have been having the same thoughts.

I think an auger is too specific and can suffer a lot if conditions arn't ideal for it.
The post hole diggers ("the scoops hinged together") look great and are efficient, but pricey and can't help you dig out old concrete.

A digging spade is good for creating small diameter holes but if the soil has rocks or concrete then it cannot dislodge these.

I am tempted a digging bar which is really just a small blade to loosen the soil. The other end is a tamper to flatten and compact the soil at the bottom of the hole. This will be the neatest method, but lots of scooping soil.

A digging bar is much more flexible in jobs you can do and will not get stuck. But it means a lot of being on your knees cleaning out the loose soil.

If you are really stuck with a mess in the hole a pick axe/mattock this will clear a large hole through anything and will make a big mess.

I think it comes down to the soil type, fix/repairing old posts or new holes. Most importantly how many holes you need to dig and how much you want to spend.

Also I find the post mix bags are just too small for 1 post, unless you are good/lucky enough to get it just right everytime. Buy bulk bags or small bags and mix your own. (depends how many holes you need)

I position the post before adding concrete with some large bricks to wedge the post before.

If time and effort are not a problem and you really want to save costs I managed to re-fence my garden at about £100 a saving of about £2500. That after 2 years a post has come loose because the concrete surrounding the post has broken a now the post can rock. It is true but I am ok with the fact that the posts will probably only last about 10 years. (lots of cresote poored down the lower part of the post) Also the posts twisting above ground is a problem.