Like every sorcerer, I also had to choose between skull trap and fireball spells. Mages don't need to decide, but they would still find this analisis useful.
I always had controversial feelings about the skull trap. In the last days I made some testing to have a better understanding. I had soon realized that its effects are somewhat different from the spell description.
Ranges. The description says: range 20, area of effect 30. But in reality it comes like this: (throwing) range 30, triggering/area of effect: 20.
Damage. The description says: 1d6 per level, but in fact it is more, probably 1d8 (per level). The good thing is that there is no cap. The damage is a magically created physical damage. It can't be catched with stoneskin, but a mirror image could usually catch it (in 7/8 of cases with a fresh casting). The damage can be decreased with magic absorbing like with the Belt of Inertia and (as 3rd level spell) even a minor globe of invulnerability can totally avoid the effects.
Triggering. This is the most problematic thing about the spell. It seems to have a random triggering, which is influenced by the total time spent in the triggering area. One fast moving target will have much less chance to trigger it, than many static ones.
Comparison, conclusion. The fireball is capped at level 10, so the skull trap can have a more devastating effect in higher levels (especially without the xp cap). This also mean an immediatel death for the caster if he is not cautious enough, so the most important thing is to use it without serious risk (at least mirror image is necessary when the caster is in the area of effect). Enemy spellcasters usually have mirror-image, so they are somewhat protected against the skull-trap, while a fireball could generally hit them. On the other hand, if the player relies only on fire-effects (fireball, flame arrow, melf's meteor, sunfire) a fire resistant enemy could be a problem.
That is all. Now it is up to you to decide which one to choose.
