Arduino's good for beginners, but I recommend ditching the Arduino libraries and switching to plain avr-libc as soon as possible. You can still use the same hardware, but you'll learn a lot more and produce better code.
When you feel you're ready to graduate from 8-bitters I suggest getting something based on the ARM Cortex-M3. They are a lot more powerful but still very easy to use and importantly, designed to be used from C. For example Luminary/TI have several cheap boards with an on-board USB JTAG interface for programming and debugging. There's also a few Cortex-M3 boards that use the Arduino form factor if you want to use shields.
>>25
Not an MCU.