Myself I've worked in embedded programming for 9 years now and have experience on TI MSP430, Atmel AVR (a couple of flavours) and will be using an ARM soon.
My suggestion is to pickup something that has some extra features in the processor like ethernet controller and CAN controller, even get two or three if you can. Embedded devices are nice to work with, but once they can talk to other similar devices via CAN or get onto a network, they can become much more fun to play with.
