I have already got the information but there is no comparison between both. I can see that PK51 is about double in price but I can't see any big advantage of PK51 over CA51.
If you read the pages carefully, you will see that the PK51 supports both the classic 8051 parts, and the Philips 51MX parts which have an 8 MB linear address space.
8 MB of 8051 code!! Suddenly I'm feeling a bit green. Time to switch to a 32 bit part.