A GPS is a Global Positioning System which is basically a device which shows your current location on Earth to a high degree of accuracy. The use of GPS is predominantly commercial for creating driving directions for commercial vehicles and figuring out the fastest and most efficient routes to save costs. The Garmin Nuvi 350 is an example of such a GPS device. Finding accommodation, food outlets, gas stations, and more is very easy with this device because it contains maps and points of interest.
A PDA is a Personal Digital Assistant or a "pocket computer" in layman's terms. Nowadays, PDAs have gained audio capabilities which allow them to be used as mobile phones too - an extremely popular example is the BlackBerry, owned by RIM. The prolific use of Blackberries by famous celebrities contributed in part to the huge and growing popularity across the nation.
The addition of GPS to PDAs has made them even more useful and popular. This means that one can find maps and directions on them too. An example of this device may be the Garmin iQue300 which holds the honor of being the first PDA to include GPS technology. Its GPS system was a Garmin and its PDA Operating System was that of Palm OS 5. An added feature of this device was that you could call up contact information through voice commands.
There will probably always be a debate on which is more preferable. Are a GPS and PDA as two separate gadgets a wiser choice or should one opt for a converged device?
Regardless of your choice, there are always benefits and drawbacks. Two separate devices will definitely work out to be more expensive. Also, while hiking or pursuing any activity outside the car, a bulky GPS will be more of a problem than a help (unless you buy a GPS suited for the outdoors, which in turn will be more expensive). In such situations you will be best suited with a GPS PDA phone. With a single device it's rather difficult and inconvenient to use both the phone and GPS at the same time. The GPS PDA phone will also have a comparatively smaller screen than that of a GPS on its own, so it may be harder to read.
All in all, it depends on your needs. If you are technophobic and want to keep using your PDA as a minicomputer and a phone; separate units are better for you. If you want to save some money and don't mind a little inconvenience, the combined device would be a better choice.