Usually you use one or the other. Thermal tape is the standard but for tweekers and the not so weak hearted, thermal paste. I may not be knowledgeable about this but using thermal paste you usually have to break in the processor for it to melt and evenly fill the gap between the heatsink and processor.
For the thermal paste, it depends how good of a even spread job you do. Plus, you have to remove the thermal tape.
The thermal paste would work better if you really get it working right. And you apply this over the area where the core would sit on. (usually where the thermal paste is)
But for safe measure, I leave the tape there. Works just fine in most cases. You might want to talk some overclockers and see their opinion.