How does the spell Siphon work for a caster who is both spontaneous and able to cast prepared spells?


If I have a multiclassed spellcaster (both spontaneous and able to cast prepared spells), can I use Siphon (Complete Scoundrel, p. 102) to regain one or more spell slot/s of any class I wish?

Or

Is the regained spell slot of the same class I have cast Siphon with?

SIPHON

[…]

You drain the charges from a wand or staff you hold to replace spells you have already cast. As long as the wand or staff has at least 5 charges left, you can expend 5 of those charges to replace a cast spell. The spell replaced must be of a level equal to or lower than the highest-level spell the staff or wand holds. If you prepare arcane spells, you can regain any one spell of the affected level that you have already cast that day; if you cast spells spontaneously, you can regain a spell slot of the appropriate level.