Change capacitance or resistance in RC circuit
It is difficult to give a definitive answer to your question because it is too general.  However some general comments can be made.  Unless your frequency range is such that the capacitor is small (i.e. measured in picofarads), it is probably better to use a variable resistor since they are available over a very wide range of values (from ohms to megohms).  Variable capacitors above around 1000 pf are not easily available and can get physically large.  Since RC circuits do not have sharp frequency response characteristics, it is probably better to "tune" your circuit by selecting values rather than by using variable components.  Fixed components have better accuracy, more stability with both time and temperature, and are cheaper.  If you do need to do precise tuning, variable resistors are probably still a better choice since they are readily available as multi-turn components.  Most variable capacitors are less than one turn.  If you can provide more information as to your application, than I and others can provide more guidance.