Powered diggers are excluded.

Which is the better choice for various soil condition in your
experience?

1. The manual auger that you twist until you get a full load of soil
to dump:

http://www.hooverfence.com/tools/adjust-augers.htm

(Needs room for handle, but can go very deep by using pipe to extend
it)

or

2. The clamshell-type post hole digger, like this dandy by Fiskars:

http://tinyurl.com/cna8js

(reportedly reaches over 4 feet)