I'm really having a tough time deciding on my fifth toon, or if I should even have a fifth toon.

DK
pros - strangulate, death grip, freeze ranged opponents with ease, tough to kill
cons - no real synergy with the priests, though 13% magic debuff could be used.

Mage
pros - freeze everyone in place for the AoE storm, decent synergy with buffs, insta cast AoE to go along with priests, nice range for sniping with priests
cons - glass cannon, would require a transfer from another account plus some extra leveling (my situation specific, obviously)

Shaman
pros - fear protection to go along with fear ward, root people in place, good burst, probably more that I'm not thinking of
cons - makes group more stationary

Moonkin
pros - decent buffs, typhoon and starfall helps position enemies, roots could be used to lock in place sniping targets, decent "tank" in PvP (big target that everyone likes to go for), could get badges with the priests by joining as a tank in PvE
cons - no spammable AoE so not sure what the moonkin would be doing during the swarm phase

Destro Warlock
pros - destro can stun for the swarm phase, hellfire could be used with the amount of healing going on though it's not mobile (I don't think), small amount of buffs, good burst potential
cons - would require an account transfer, doesn't feel like a great synergy between the teams

Fifth Priest
pros - more healing, more AoE, perfect synergy which could enable me to use more priest specific abilites since I'm not working about diversity
cons - lack of additional buffs

Playing five toons instead of four
pros - more damage, more coordinated burst
cons - more responsibility in BGs, have to drop a member to have a friend join, can't gear in PvE quite as easily (have to rotate)

The swarm phase is probably the most important. A mage or fifth priest probably handles it the best. A DK can help with the hunters that seem to be giving me the most problems.

Any ideas?