I use PIC chips for my robots. I have an inline debugger and programmer, which are the most expensive pieces. After you purchase those parts, everything else is available.
If you constuct robots and have a general idea what kind of requirements you have, you can usually go onto the microchip website and get 'samples' of your parts for free. They're free microcontrollers, and they're expandable to whatever you can think of. Microchip most likely has them in stock. If you can't get them for free, they generally cost under $10.
My friend owns a BASIC stamp 2, but they tend to cost quite a bit more. They are better for beginners, because they have all the required support electronics on board.
It also depends what programming language you are working in. If you know C well, go for the PICs. If you know BASIC better, go for the stamps.
Also like JesseWelling said, it depends on applications you plan to use it for.