Enervation is stronger.
Both Enervation (4th) and Disintegrate (6th) are solid options.
Enervation has a few advantages over Disintegrate:
- No saving throw. This is seriously good, as most demons have good fortitude saves, this is a great advantage to have;
- Negative levels make the target easier to kill. The debuffs caused by negative levels are nasty and difficult to remove;
- There is no amount of hp that can save you from the penalties, so if you hit the target (be that an imp or a balor), they take the penalties;
- More casts per day, being a 4th level spell, you will have more slots available than 6th, and you will have more room to work your metamagic feats.
However, Disintegrate has several advantages over Enervation:
- Can instantly kill the target, this saves actions from the rest of the party. With 30d6 damage, this is nothing to joke about, specially if you mix metamagics;
- Will affect creatures immune to negative levels. Even if you have a thanatopic rod or metamagic, some creatures will simply ignore the effects of negative levels (like constructs).
- It has a much longer range, being a medium range spell, it will reach targets 250 feet away, while Enervation will hit targets at 60 feet away.
Keep in mind that at level 15, demons around your average party level will hardly be killed by a single cast of either of those spells. You should take a look on their sheets and check for vulnerabilities:
- Nalfeshnee. Strong fortitude, low reflex, Enervation is better.
- Marilith. Really strong fortitude, low will.
- Seraptis. High touch AC, high fortitude.
- Shemhazian. Moderately high fortitude, low reflex.
- Vavakia. High fortitude, low reflex.
As you can see, the lowest fortitude save is +19 around your level, which makes Disintegrate an unninteresting spell to use, as they will only take 5d6 damage per cast most of the time, and you have better spells to be using your standard action on.
