I have used both and here are my thoughts:
A dedicated (Garmin) GPS receiver has a bigger screen and is easier to enter addresses and info. You can set custom points of interest (POIs). Map updates for my Garmin are $80! Most GPS units will log trip info such as avg speed, total distnace, etc.
On a phone/PDA, you have the advantage of being connected to a the data network. The ability to search for places is much more flexible. The maps are always up to date with new roads, etc.
The accuracy of both appear to be similar.