You mean which microcontroller to get? That is...a deeply personal choice :-P What you'll end up finding is that PICs, Atmel chips, Arduinos (based on Atmel...) etc all do just about the same thing, but they are programmed slightly differently and have different user communities.
I like Atmel chips, especially because great libraries already exist for them (Arduino and webbotlib), plus the user community here is awesome. But at the same time, I learned on Atmel chips, so...maybe I'm a bit biased.
To start with, if you're going Atmel, the ATMega328 is a good one, as is the ATMega644. Both have the same capabilities in terms of computation, but the 644 comes in a bigger package with more IO and more memory (64K compared to 32K). If you're using webbot to program them, the code is essentially the same, although the pinouts are different.
Interestingly, the setup circuits are almost identical as well (except for pinout, which changes). I've played with both, I like them.