It depends is the answer...
In short if you are budget minded AND you are looking for a lower end computer AMD XP 2200 or lower, then you will get a MUCH better $$/CPU power ratio w/ AMD... If you are looking to build something a little more powerful, and have more money, the P4s are a better deal (as opposed to Opterons). This seems to be generally accepted--but I guarantee that someone will contradict this.
The other difference is the heat that you deal with... AMD chips run hotter--thus they require noisier or more complex cooling schemes...
my two cents