The higher the value of the capacitor, and the lower the value of the resistance (the knob is a potentiometer, or a variable resistor)the more highs the tone control will eat. So if you want a darker tone, go with a bigger capacitor (or a lower-value pot). For a brighter tone, go with a smaller capacitor (or a higher-valued pot).
When the tone control is set to is brightest setting, the resistance of the pot will be at its maximum. If you go with a lower-valued pot, that will simply limit how bright you can possibly go...