Steering throughout the corner (entry, middle, exit), because it is the main component of the vehicles ability to grip the track. With foams, a softer front tire gives more turn-in, and mid corner steering. A harder front tire would cause you to push more.
The main difference between foams and rubber tires are their consistency. Foams grip, but once they lose it, you can't really 'come out clean'. Not that you're going to spin out, but that corner is obviously going to lower lap times by .2-.5 seconds on its own. Rubber tires break traction more evenly so basically your car kind of drifts. Foams don't really drift, they push or they oversteer. Also, you have to drive them differently. There are two ways to drive foams and driving rubber tires on micros is right in between the foam driving techniques.
Foams, you can either drive relatively hard, where you have a setup and matching driving style that can easily break traction (slide a bit around corners); or you can very steady. If you drive steady, you have to be very smooth on the throttle. If you are committed to a turn, you can't really change throttle position too much or else your car will push (if you speed up the car, or steer too much -not necessarily oversteer- but the front end just bites in too much and sends you on a line that totally kills exit speed. Rubber tires are in between. If you push too hard, all four tires will slide just a little. If you check your speed in the middle of the corner, your front end won't bite-in and turn toward the wall. Overall, foams are faster almost all the time for me, unless things are really slick.