It also depends on the type of soil you are working in. IF the soil is normally clear of stones/rocks that could either prevent a post from being driven or cause it to deflect and go crooked, then a post driver can be a problem.
If you want your fence lines to be perfectly straight, then a PHD is the better method. If slight variations don't bother you, then a driver is often faster and less labour intensive.
A posthole digger is a versatile tool because of the range of auger sizes it will drive. I think I still have a 24" auger around that is designed to plant shrubs and trees in shallow holes. The best posthole diggers are the hydraulic ones. Being able to reverse them instantly and unscrew yourself from the hole is a huge timesaver. I've owned three-point hitch - PTO driven ones and the hydraulic model. Once you go hydraulic, you will never want to use a 3 pt digger again.