In my book g/cm is more valuable to consider than rpm. Many people go for the highest rpm motor they can stuff into a car without considering the g/cm.
Basically it's the torque of the motor. The higher the torque, then the faster you get to top rpm and the more weight and/or magnetic downforce you carry along with it. Some argue that higher the torque, the better the brakes and control, also. I'd agree with control, but not always with the brakes argument.
SlotIt motors have a high rpm, but a pretty average torque (about 100gcm). This means that it takes longer for them to get to top speed down a ond straight in comparison to, say, a lower rpm Ninco that is stuffed with higher torque from NC2 upwards.
Ratios depend upon your racing likes and your magnet/weight set-up, and your layout rather than a universal rule (such as pinion/gear ratios).
The best easy to get all rounder at the moment is the Fly racing motor that has about 24000rpm and a controllable high torque that easily outstrips SlotIt (that has a much higher rpm).