If you go for Atmel then the ATmega8 is old and outdated. Use an ATmega48, ATmega48A, ATmega88, or ATmega88A. They almost provide all ATmega8 features (ADC reference voltage can be an issues) and more, e.g. they have a debug interface.
I believe the current models are the PA models, which also includes Picopower (low power).
Original Atmel programming and debugging hardware is ridiculous expensive, breaks easy and won't be serviced by Atmel.
Pricing for the AVRISP Mk. II and Dragon is OK, the higher models are indeed expensive. Original iteration of the Dragon was quite fragile, current version is better, haven't heard any complaints about the AVRISP.
The latest Atmel IDE (Studio 5) is so severely broken that it makes you vomit. But Atmel claims they know what is good for you and you should just go with the program.
Don't like AVR Studio 5 either, but getting an open source environment setup (eg. with Eclipse and GCC) seems to be easier for AVR than any other uC. Not sure if this is because or despite Atmel's efforts, though.
Atmel loves to announce stuff for immediate availability when having only slideware and only two years later the first parts show up, but then really broken.
As opposed to the MSP430 Launchpad, which was shipped right away when it was announced?
Power usage of the MSP430 isn't that special anymore compared to the more recent Atmel/Microchip offerings. Depending on the exact application, any of them can be the best.
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On the other hand, the chip included with the launchpad has 1/4 the ram, 1/4 the flash, and half as many IO pins as an ATmega8, and the MCU power is not a major consideration for most robotics projects (seriously rendered irrelevant as soon as you turn on a motor.)
I was so confused when I first read this until I realized I have the booster pack for the LaunchPad so I use the MSP430G2452IN20 instead of the stock MCU MSP430G2231/MSP430G2211
OK, still 1/4 the RAM (I think the original Launchpad chips were even 1/8th of the RAM) and less I/O pins. To be fair, you should compare something more powerful (probably not available in the Value line) to the ATmega8, or compare the Value Line chips to some ATtiny. I would also recommend something like the ATmega88PA if you go with Atmel, not the old ATmega8 which lacks Picopower or debugging.
Nothing wrong with choosing either families of MCU, just pick one that has the necessary resources and where tools and expertise is available. The only difference may be the amount of example code available, this appears to be more limited for the MSP430.