This isn't so much a scenario of apples versus oranges but apples versus antelope.
Several questions to consider:
1. What are the power levels of the magical styles deployed by the two magi?
Gandalf the White would leave any mortal practitioner of voodoo hanging from their toes since in the Tolkien mythos Gandalf isnt a human but an Istari, a being rather akin to a weak Loa.
2. How fast are they able to cast spells?
A mage capable of rapidly casting spells is going to have an advantage over the mage who has to prepare half an hour, drawing circles, verve, chanting, getting their perspective of reality warped by drugs, sweat lodges, etc
3. How versatile are their spells?
The voodoo magi isn't going to do very well against a pyromancer, since his spells require physical components from it's targets ala fingernail clippings and hair. A zombi will be toast, and cursing is too slow. Against a necromancer, the voodoo ongan/ashkar will propably have an even fight, so long as it sticks to cursing and such and not who can raise the dead better
4. Is their a way for Magic Duels?
The Mage: the Ascension game had a magical combat system called Certamen. This was a formal ritual/spell where the two combatants squared off and contested their magical skill in a duel of wits, perception, and intelligence with a minimum of collateral damage. This also allowed magi of differing paradigms to duke it out, such as a God fearing Clerical Magus going toe to toe with a brass and steam steam punk technomancer