It really depends on your needs and what you are trying to do. If all you are doing is blinking some leds, it doesn't matter much. An humble 10F200 can handle 6 leds by charlieplexing and a 12F508 can handle 20 leds. I've build an LCD signboard using a lowly 16F54 ($.55 US from Newark). The basic message software takes a bit over 100 instructions out out of the available 512 lines. There's room for 6-10 more messages. Yes, programming can be a pain with only a two-level stack, but it's also fun. Mid-level pics add more features such as interupts, deeper stack, and more timers and other peripherals. 18F add still more features including hardware multiply and a faster clock. While the 16F running at 5 MIPS can only do crude video, a 30F at 30 MIPS becomes interesting for video. I am currently interested in what a 30F or 24F can do with computationally intensive problems such as guidance, navigation, and control.