If you are working for an electronics company, you may be able to obtain samples from the manufacturers. Have them mailed to your work and you can try out different uCs for free.

I am also building a robot, but I have chosen a PIC18F452 as the controller. Microchip sent me one each of 4 different uCs to "evaluate". I also requested 3 TC4425 motor drivers, which I will be "evaluating".

As far as what chip to choose, I would recommend getting as much I/O and on board circuitry as possible. If your going to use a Sonar sensor, it would be nice to have a Capture/Compare Timer. If you will be driving motors, PWM mode timers are good. If you have analog sensors, you'll need A/D converters. And of course you'll want plenty of digital I/O. Speaking from experience, if you have 24 I/O ports, you will quickly need 25, then 28, then 32. So you may want to plan for the future and look for some kind of serial port that can be used as a master/slave bus.

It all comes down to your design, so plan it out, add up all your I/O ports, and double it. Then start looking for a chip that has everything you need.

Good Luck !!! :)