Hi, I want to make a differential steering robot with pan and tilt (so 2 dc motors, 2 servos) and, I wonder, do I need an extra motor control board? I see places that say that with the servo library I can control 12 servos but, the documentation says it's not that accurate with more than 2. And, it seems the servos want very short pulses, while motors need long ones for PWM, making it harder to share timers (I imagine). Well anyway, I'd like to be as accurate as possible (it's just hobby, but still..), what do I need? Is an Arduino enough by itself, or do I need the arduino motor shield? How about the Pololu micro serial 8 (can't post link), which seems to be cheaper ($20 vs $30) and has more capability? (or is using serial more work, or slower?)
You will need some kind of external hardware for the motors, if you want to control speed and direction then one of the H-Bridge shields would be a good choice.
I see places that say that with the servo library I can control 12 servos but, the documentation says it's not that accurate with more than 2.
Are you looking at the documentation for the Arduino Servo Library? The arduino library is just as accurate with 12 servos as 2.
And, it seems the servos want very short pulses, while motors need long ones for PWM, making it harder to share timers
The servos and motors do not share the same timers. On a standard Arduino board you can control the speed of up to 4 motors using analogWrite while controlling up to 12 servos.