It depends really what you want to do with them, PIC's are small and cheap and use a RISC architecture - this means they are fairly simple to get to grips with. The Motorola processors are probably more powerful, but cost more and are less simple to use.
Using something like a PIC 16F628 will cost you very little to get started, and there's lots of help available. PIC's are cheap enough and easy enough to use to replace things like a 555 timer - it's not as cheap, but it uses a lot less parts and is far more accurate and versatile.
For what it costs I would suggest you try PIC's, and see how you get on.