You should choose depending on your particular discipline:
1. For cross country
Your choice should be based first and foremost on whether the terrain is dry or slippery, muddy or hard.
2. On an Athletics track
Your choice should be guided by your discipline and the grip you need, keeping in mind that your sport may have some length restrictions.
Grip or comfort?
On synthetic tracks, you will usually have the choice between 6 mm and 9 mm spikes. 9 mm spikes will provide better grip of course, but you are more likely to feel them underfoot, which may become uncomfortable, especially on long runs.
On dry or rocky terrain
9 mm spikes are generally sufficient and will provide you with extra comfort, especially on long cross-country runs.
On muddy, slippery terrain
On muddier ground, you will be looking for excellent grip with longer spikes (up to 15 mm). According to your stride (heel or midfoot strike) you could try wearing longer spikes at the front for an optimal compromise between grip and comfort.
What about jumping and throwing?
For the most efficient push-offs, you could wear spikes up to 12 mm, which is the maximum length allowed in competitions.