The Priest has the advantage of an opponent not really being able to start gaming his opponent by his mage card. Paladin certainly has cool tricks, but you have to play into them to maximize them. Which means predictability, a player can start to guess what youre going to do and try to "deny valor" making it more of a risk/reward sort of thing. Priest on the other hand isnt reliant on a narrower secondary mechanic or minigame. By its very nature, Valor is telegraphed, the opponent will know how you generate it and know how much you have at any given time, and what spells you might be using it on and probably even WHEN you will in some cases. Which means the Paladin can have his own tricks stiffled or be forced into tough decisions by his opponent.
When an opponent sees hes playing a Paladin, he can already start accounting for Valor in his own strategy, when the opponent sees hes playing a Priest he cant assume as much. (much how the Forcemasters biggest weakness is simply that she is a Forcemaster and can anticipate much of her spellbook and gameplan already.)