Dispel Magic SEIGE check

[ThrorII]

According to the PHB (3rd printing), page 70:
The caster must make a successful intelligence check against each spell, effect, object or creature in the area of effect to succeed.

Is the intelligence check modified by the opposing caster's level, the opposing spell level, or both?

For example: A 5th level wizard, with a prime intelligence of 16, is attempting to dispel a "transmute flesh to stone" spell (6th lvl), which was originally cast by a 15th level wizard.

Is the SEIGE check

d20+5 (wiz lvl) +2 (int) vs. 12 +6 (spell lvl)

or

d20+5 (wiz lvl) +2 (int) vs 12 +15 (caster lvl)

or

d20+5 (wiz lvl) +2 (int) vs 12 +15 (caster lvl) +6 (spell lvl)

In the end, what is more important: the caster level or the spell level?

[serleran]

Caster level.
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