I guess this comes down to which class gives better support to caster dps in raids - paladin or shaman. Paladins have a nice mana regen blessing, as well as a threat reduction buff. Shamans, at least that's what I gathered, are better at supporting rogues and fury warriors.
This is just my opinion, I'm not a very experienced raider so I might be wrong. I'm thinking about rolling a caster (haven't decided which one) and I would just like to know which faction class helps the caster dps more.