I'm a Paladin sword Fiona myself, but here's what I have found to be the case.
Dark Knight=extreme offense
The only differences between their two skills is
Aura that increases stamina for self and party members close by, keep in mind this buff will NOT stack with that of another Dark Knight.
Two passive attack buffs, one of which increases your balance
An attack skill that does a pretty decent amount of damage and freezes the target(s) for about a second. Can only be used once in 2nd stage trans
Steal chain, absorbs fairly small amounts of HP from enemies when you use a smash and gives you a very tiny attack boost. The final skill in this chain will slow enemies.
Aura that generates a small amount of SP slowly for you and any party member close by. Once again, it will NOT stack with the buff of another Paladin. However, the fatigue and vigor skills do appear to increase the SP generation, albeit not by much.
Two passive defense buffs
The skill conviction, which is a skill that can only be used once in 2nd stage trans. This skill is based on your defense, so a Fiona will be able to absorb more damage with Conviction then a Lann, Evie or a Karok could. That being said, as a Fiona I only use it to cover any mistake I may make or to block attacks that my shield cannot.
The grasp chain, which functions similar to the DK steal chain. The differences is however that it doe not buff you but instead deals slightly more damage, the final skill in this chain will petrify instead of slow. Note that this and the DK skill do NOT slow/petrify bosses.
In the end, neither will make or break your character as your trans can ONLY last at most 2 minutes once an hour. --- Name: Rain Friend Code: (will put in when not lazy)
well, Paladin used to be considered better before the revamp and Stage 2 due to Petrification, actually doing more damage with Grasp, and the damage cap being easily reachable anyway until they started ramping up boss' DEF, so it's only right that Dark Knight gets to be better for a while
but **** efficiency, Karok turns into Megatron for Dark Knight Stage 2! --- Note to self: Cannon highly effective against flesh. -Mimiron
About the balance between the two, keep in mind the balance was also in favor of Paladin previously. Now it's simply shifted to Dark Knight. Time will tell if they finally strike a truely perfect balance for third stage or 4th if there ever is a 4th.
Either way, neither choice matters in the end. You'll still roflstomp pretty much everything regardless. --- Name: Rain Friend Code: (will put in when not lazy)