Heat Rocks: I do not recommend these, especially older models. In the past reptiles have been known to sustain serious burns and many experts recommend avoiding heat sources with which a reptile can have direct contact. They also are not as effective at heating the ambient environment as some other sources, and I think with safer and more natural options such as overhead heating, these are best avoided.
Heat Cables: these are a fairly new addition to the market and offer a flexible heating alternative, and can be used in the tank, under the tank, or in breeding racks. I would probably stick to using these outside the tank.