Rental equipment selection for digging 52" deck footings?
Having dug footings before by hand (2-man auger, clamshell digger and shovel) I would definitely suggest the Dingo for this project. I did 5 with the auger and 2 with clamshell (because it was near utility lines) but 20 footings is a lot and you should keep this simple and quick, letting the machine do most of the work.

Note- clamshell post-hole digger example:



A clamshell is not better than 2-man auger, it's a lot more work but you have to hand dig near utility lines. The 2-man auger has 4 handles and the motor in the middle, one man operates from one side and the other stabilizes as it digs. It's fast but it's a lot of exertion, especially for as many footings as you're talking about.