I have not seen any PDAs with wireless modems for them. So if there are some, there should a big competition between theses two technologies.
But cell phone is reliable because it is wireless for Internet, its phone (yeah I know there are some PDAs with phone features in it, but.. not good design), it is faster then PDA I guess.
PDA is good: you will have all your word documents in it, you can pretty much information in it comparing to digital cell phones, display is more adaptible then the Internet cell phones, etc.
PDA is obviously going to be more expansive, just look at the price of the Palm VII for example. But it has more features than a cellphone. For example, it is much faster to type text with a PDA (You can even connect keyboard).
Also, with a PDA, you are able to save data, transfer it back to your PC, and so on.