As you can guess, this is a subject that people argue about for hours. As far as I can tell the Athlons are
(a) Quicker than a P4 for a given clock speed (i.e. Athlon 1.4GHz is faster than a P4-1.4GHz)
(b) Cheaper than a P4 of equivalent performance.
Seems to me that an Athlon is the way to go if you have a choice between the two - if you're building your own machine, for example - but if you're buying and see a good deal on a P4 then I'd just go for that. I doubt that there is enough of a difference to care about unless you're a techie worried about the performance of the machine on the limit.
FWIW, I've just acquired a P4 which I'm very happy with.