When I decided to get started with microcontrollers, I downloaded a bunch of data sheets, shopped around for programmers, looked at the compilers, prices of various chips. I went with Atmel, as it seemed like the best value all around. I don't know 'C', or much about compilers, so figured I'd be using Assembly. Atmel chips have 130 instructions, and 32 registers, PIC has 32 instructions, and only one register. Figured PIC would be kind of a pain to program in assembly. The one thing that really made me jump though, was the programmer. For $50, I got the AVR Dragon, it's USB, and covers most every programming mode, debug, and supports pretty much every chip they make. Does a lot more than I really need, and half the price of what I was looking to spend. The free software is great, compared to what I was reading about PIC (got to buy, or it's a good idea to purchase, unless you have a lot of patience).
For most hobbyists, doesn't make a huge difference. Most of these chips pack way more power and features than most will ever need or use. Speed? I usually need to slow things down, lots of delay loops...