The question here "which is best PIC or '51" is ridicolous. For some applications it is one, for some the other.
Also, there has been threads on this forum where someone tries to cram an application into a '51 for which the '51 is not suited just because (s)he had a '51 or '51 knowledge.

Sometimes the '51 is too much, sometimes too little, sometimes just right.

One reason to lean towards the '51 is the multivendor availability, if Atmel goes on allocation you can buy Philips, Dallas or a host of other brands.

Have fun,

Erik