I normally consider things in terms of the different situations you'd encounter and what the benefits are for each.

Non-elites = toss-up. Priest can heal through a lot of non-elite damage to the mages. Pally can just aggro them all and consecrate while the mages AoE, then heal afterward.

Regular elites = toss-up. Single mobs = doesn't matter. Multiple elites = roughly the same as non-elites but pally has a harder time keeping aggro off the mages and staying alive long enough to kill things. Conversely, priest would drop fast if multiple elites aggro'd him. Really depends on the elites. For example, full pulls in Steamvaults would be very difficult trying to AoE as there's just too much damage on the pally (even protection spec) and too much damage on the squishies otherwise.

Boss elites (or similar strength) = priest. For most of this type of mob, as you're gearing up a pally just can't pump out the healing needed. And if the pally is tanking, stopping to heal is bad and only compounds the healing problems. Most folks that run this kind of group, though, probably won't be doing bosses or tough elites without a tank.

Instance runs = priest. Most instances can be done with a pally but you again hit the problem where trying to do two things at once with the pally is a tough act. With mages, you just blow everything up and AoE heal. Again, with instances where you can CC everything (think live strat or Hellfire ramparts) you can use either priest or pally as you limit your exposure.

PVP = not sure. My experience in PVP is fairly limited as I've been mostly enjoying leveling and PVE to see what groups work best for what I mostly like doing. I would think that this would be another case of toss-up. Priests have more/bigger heals and group heals but paladins can withstand the focus-fire most guys will eventually bring against your group. Focus-firing a paladin is ineffective when they're invulnerable. When they're not, they're in plate with resist auras and hard-to-interrupt self-heals. They also have that nifty stun. Concentration aura = win for your mages for casting through damage.

So, tough call. I look forward to seeing some more detailed responses from some folks with a bit more experience in PVP than me. I'm sure my ideas will adapt as I level (only 42 at the moment) on Magtheridon.

Either way, best of luck!