Sorry, I meant Dark Knight

Right now I have a Dark Knight, White Mage, Summoner, and Dragoon. Should I change my dark knight back to a regular knight? Whats the advantage to either?

Knights have both good offense and defense, and can use the best weapons and armor in the game. They can also use level 1 magic and Guard, which lets the user take all damaged aimed at party members with low HP.
Dark Knights are the same, but can't use white magic and have a special skill:Souleater, which can attack multiple enemies. Their armor and weapon selection is limited, which can make them weak later in the game.
Personally, Id pick knight.

dont change it or do as you please

I like the Dark Knight. If you've ever played Final Fantasy IV you'd see that cecil's Dark ability is extremly helpful...until he becomes a paladin....

But I personally like Dark Knight better.

Knights are slower, but as they progress, their defense and offense outweigh the benefits of speed twentyfold. Moreover, the Knights are able to wield BOTH Excalibur and Ragnarok, which really help against the last five bosses in the Underworld.

i never liked knights that much, but damage wise id go dark knight, their souleater can be pretty devistating from my experiance