I have an 80 ret paladin, 80 mage, 80 rogue and 80 warrior. The ret paladin is by far the funnest class. His dps is amazing and i can switch to prot for instant que's as mentioned before. The mage comes in as a close 2nd, its fun being able to attack shit from the back field, especially since ive played melee characters for so long. My rogue is a nightmare, i dont enjoy them at all. i played a rogue up to and through TBC, wotlk i hated every minute of it. Im hoping cata brings him back to life.
About your concern playing a paladin since you already have a dk. I was worried about the same thing when i started my warrior. I thought, damn the same loot all over again, and how different could the attacks be anyway. I was wrong, the loot had no effect and i was just as excited to get the same shit over again for another character, and as for attacking, it was completely different as well.
So i say, go paladin..and if you dont want to do that because of your dk, go mage.