When casting a Runestaff spell from the spell list of one class, but using a spell slot from a different class, which caster level do I use?

Question

A runestaff (MIC p. 223) allows a character who's attuned to it to sacrifice one of their spell slots to cast a spell of equal or lower level from the staff. This allows characters to cast spells that they don't know, though they must still be spells from their class spell list. From MIC p. 224:

By expending a prepared arcane spell or arcane spell slot, the wielder can cast a spell of the same level or lower from the runestaff’s list, as long as that spell also appears on the wielder’s class spell list.

Per my previous question on this topic, it's possible for a multiclassed spellcaster to use a runestaff to cast a spell that's on the spell list of Spellcasting Class A, but using a spell slot from Spellcasting Class B. For instance, a multiclassed Duskblade/Wizard using a Runestaff of Enchantment could use expend a 1st level Duskblade spell slot to cast Sleep, a spell that appears on the Sorcerer/Wizard spell list, but not on the Duskblade spell list.

Normally, a spell cast via a runestaff uses the character's caster level. Again from MIC p. 224:

The spell is treated exactly as if the wielder cast the spell herself, including caster level, save DC, and any other effects related to the spell.

However, in the case of the Duskblade/Wizard above, the character has two different caster levels. When casting Sleep from a Runestaff of Enchantment, does this character use their Duskblade caster level, or their Wizard caster level?

Answer 1

I have my doubts about the answer to the linked question, precisely because of this question: if you were allowed to do this, the answer to this question ought to be clear. It isn’t.

But anyway, I would say that if you are using the answer to the previous question, you would use the caster level of the class whose spell slot you are using, not the class that has the spell on its list. My reasoning is that having the spell on the other class’s spell list allows you to use the runestaff at all, but the spell slot is the thing actually powering the spell—therefore you should use its power, that is, its caster level, to determine the spell’s effect.

This is far from clear or definitive, however; it is merely my ruling.

Answer 2

Assuming that the we are using a ruling based on the answer to the linked question:

I can see a fair argument to allow you to choose which class's caster level is to be used.

Per the Runestaff entry quoted in the question (MIC p. 224):

The spell is treated exactly as if the wielder cast the spell herself, including caster level, save DC, and any other effects related to the spell.

It gives you no mechanism by which you should choose what class you are casting the spell with. Of course, you can't mix and match classes, since it is treated "as if the wielder cast the spell herself", and you cannot normally mix and match spellcasting in this manner. So, for example, as a Wizard/Cleric, you can't use Wisdom for you save DC AND use your Wizard caster level. Either use (Int and Wizard level) or (Wis and Cleric level).

Per the Staff entry in the SRD:

Staffs use the wielder’s ability score and relevant feats to set the DC for saves against their spells. Unlike with other sorts of magic items, the wielder can use his caster level when activating the power of a staff if it’s higher than the caster level of the staff.

It also gives you no mechanism by which you should choose what class's caster level to use. I'm not aware of any rule that stops you, as a Cleric 1 / Wizard 19, from casting Miracle from a Staff while using your Wizard caster level. Assuming there is no such rule, I don't see why Runestaves would work differently. If there is such a rule, then I see a strong precedent to rule that either:

1. The answer to the linked question is wrong. You should treat each class in isolation when using runestaves.
2. You should use the caster level of the class which has that spell on its spell list. However, in this case, I would prefer to rule as KRyan did in the accepted answer.