This is the biggest difference between micro's
8051 is linear, and along with the four ports you can actually connect any device to a memory location... This makes the micro more of a full processor... Sometimes a good thing to have, the downside is that you lose two bidirectional ports.
The PIC series have paged memory, and not all the same size... However they have oodles of peripherals on board.. You can't use the memory as i/o ... Its just a micro...
One upside is the new parallel port... You can use this similar to a data bus and connect to other devices... LCD's come to mind..
The ATMEL AVR is similar but has linear memory... It too can boast tons of internal peripherals... But also can't map memory to i/o..
I think ( Its my opinion ) that the paged memory is the biggest issue on the PIC.. ( I'm talking mainly about the 8 bit systems)