PICs are a nice choice. I use them to do my robots and such. It supports several different types of PIC chips, which is a plus. Those microcontrollers are nice, because you can get many for free off of the microchips sampling page. This allows you the flexibility of choosing your unique chip for each robot project and they aren't extremely expensive.
My friend got the BASIC Atom Pro microcontroller and now faces moving chip from bot to bot, because they are $60 each. It's a nice choice, I'd stick with it.
If you are fairly new with programming and robot construction, look at Microchips Mechatronics development board. It's $150 but it got me started and as good as I am today. It's pricy though. I got mine from Batelle, because they wanted me to build a robot for their company.