Main differences:

PIC's have a hardware stack, 8 deep I think. So not that good for C, although several decent compilers do exist. But this does save RAM.

PIC's do all non-branching instructions in a single cyle. So working out time to execute a routine is easy (they also tend to be pretty quick).

PIC's program memory is banked. Yuch. When you cross a boundary you have to change some page bits somewhere. This can really confuse you when things suddenly stop working.

PIC's often have a bit more RAM (about 370 bytes) but they don't have that wonderful bit addressable memory that '51's have. So a bit of a toss-up about memory usage.

You can get a complete IDE for PIC's free from Microchip, and it's not bad at all. Also has a good simulator.