hi,
Most of the PIC`s have small internal EEPROM for temporary storing of data in case of power down which is not seen 51`s. PIC`s having only 40 and odd instructions is very suitable for small projects. There are many 51`s cores availble with many vendors which is very useful for FPGA or ASIC implementation. But i havent heard any cores of PIC available(if anybody know please let me know). Cores for FPGA/ASIC implementation is used to develop application orientated IC`s. Presence of watch dog timer is an advantage in PIC.

There is also memory constraint in PIC ( both data and program memory) for large applications.

Finally ur choice of controller should be on application,cost,development tools, complexity of project and other such things.

regards
-sathish