extreme.aly (and everyone)
I choose my microcontroller not by 'how good is it' but by what features I need and programmability.
For example, if I want 8 sensors and 22 servos on my robot, this really narrows down my choice of microcontrollers . . .
Sometimes you might want ultra speed, or ultra cheap, or ultra small.
I recommend reading up on the features of the microcontrollers, and deciding what you need on your robot, before buying a microcontroller
As for programmability . . . some microcontrollers require expensive compilation software or programming hardware that I tend to avoid . . .
And then there is the amount of source code and support community size out there to consider, too.