Hm.
Disintegrate has a Fortitude save.
And an 11th-level Fighter, assuming Con 16 (14 + 2 item) and a +1 cloak of resistance, has a Fortitude save of +11. (Or +13 if he has Con 18 and a +2 cloak.)
The save DC for
disintegrate is 16 + IntMod (and Spell Focus: Transmutation, if any).
Assuming a Wizard with Int 21 and without the Spell Focus feat, this equals a DC of 21. (Or DC 23 for a wizard with Int 23 who has the feat.)
If both characters have stats and gear of roughly equal power, the Fighter will make his save on a 10 or more - i.e., 55% of the time.
Assuming the Fighter is a human, half-elf, elf or orc, anyway.
If dwarf: 7+ or 70%; if gnome or halfling: 9+ or 60%; if elf: 11+ or 50%.
In case the Fighter saves, he takes only
5d6 damage.
If we go with the aforementioned 55% save chance, this means that the Fighter takes 5d6 55% of the time and 22d6 45% of the time. So on the average, the Fighter will take a bit less than 13d6, or 44 points on the average.
Average hp for a Ftr11: 98 (@ Con 16) or 109 (@ Con 18).
44 hp equals 45% (or 27% @ Con 18) of the Fighter's hp.
Hm. I'd say that the
scorching ray is the more damaging spell, assuming the target is a tough one (good Fort save, high Con, big HD).
For mopping up Rogues (and Bards) or Wizards/Sorcerers, though,
disintegrate is deadlier.
Disintegrate also has the advantage of being a higher-level spell. A
scorching ray is still 2nd level even if Quickened - so even a
minor globe of invulnerability defeats it. Also, fire damage is easier to protect against than
disintegrate effects and
disintegrate requires only a single attack roll while
scorching ray requires one per ray.
And it kills everything it reduces to 0 (not -10) hp - and reduces them to dust (not a good thing if you wanna be
raised).
But on the other tentacle, the
scorching ray we're talking about here has the distinct advantage of being
Quickened!
So what's the more powerful spell - a plain old
disintegrate or a Quickened
scorching ray? Heh. That, y'all have to decide for yourself.
edit - One more thing... I have to say I'm glad that
disintegrate was nerfed (from instakill to huge damage) in 3.5.
