I've competed, photographed and have been involved in organization of rally. Bring both lenses. On the competitive stage roads, you'll need long zooms to keep yourself away from the cars and flying rocks and dust (or mud). But if you have access to parc fermé or other car display or service areas, then shorter lens will come in handy.
Don't underestimate the danger or the amount of dirt and dust. The nature of the sport means that if anything goes wrong, you will be far from help.
Remember that faster cars are released first in the start order, so there is not much chance to practice. Pick your spots and angles using the course opening cars or other landmarks. When the cars go by, you will only have a few seconds to shoot.
