A smaller wheel will be lighter. Lighter wheels mean less rotating mass (which means faster acceleration and braking), and less unsprung mass (which means the suspension will be able to react faster and will keep the tires in contact with the ground better. If the tires are on the ground, they can then provide traction. Lighter wheels therefore mean you get better traction). A smaller wheel also allows you to run a smaller diameter tire. This means you can effect the final gear ratio, and potentialy make the car accelerate faster.
I have been told that larger wheels are better at providing a higher G-load at high speeds...extreemly high speeds....speeds higher than our cars can go. I can't explain why because I just don't care, but this is why high end exotics/real race cars have such big wheels. They are willing to give up the benefits mentioned above to achieve higher sustained cornering G's, something that is not important at an autocross.
The only reason to run a larger wheel is if the smaller wheel causes you to want to start shifting between 2nd and 3rd because you are running out of gear in 2nd. I can't imagine a course fast enough to cause this in the 3, but you may run into this problem with other cars. IMO, the 3 is geared about perfectly when you have slicks.