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Which do you think would be more useful/powerful to have in your party, a 10th-level half-celestial paladin, or a 10th-level human paladin and an 8th-level cleric? Why?
I voted for Cleric and Paladin, but that was assuming you meant ECL 10 Half Celestial Paladin. Is that accurate, or are you talking a Half Celestial Paladin 10 (ECL 14)? If the latter, I think the 4 ECL difference easily makes up for a cleric 6 levels lower.
In a straightforward fight, a Paladin backed by a Cleric will have 3 or 4 times as many buffs on him as the Half-Celestial Paladin (nominal ECL 14). The effectiveness in combat of the two would be quite comparable.
In unusual situations, the added actions, more HPs, and greater variety of special abilities by adding in another character is going to be more useful.