I'm in a very similar situation with PLD and DRK.

I leveled DRK as soon as HW was released, and have it at 60 through finishing msq etc. I'm now working on PLD and using DRK for my expert roulette.

I really enjoyed leveling DRK, however when doing the exact same dungeons as a PLD, I am finding them a whole lot less stressful and a whole lot easier. Whilst DRK damage is awesome whilst tanking, especially compared to PLD, I feel that the inclusion of dark arts effects on defensive cooldowns was a mistake. I felt that whilst leveling, a lot of the DRK cooldowns were quite lacklustre compared to PLD, unless buffed. It seems silly to need one tank use resources to buff their cooldowns.

This brings me onto the second point, resources. I feel that the MP management of DRK is just an unnecessary layer of complexity. The DRK abilities just don't seem to mesh all that we'll with recovering MP and abilities, particularly key ones such as grit, cost way too much. PLD doesn't suffer quite so much in this department as flash is relatively MP efficient and shield/sword are pretty cheap. MP management as DRK just seems like an extra thing to worry about, but the effect is to bring it up to PLD/WAR standards.

Looks wise, it's a wash. The new DRK artifact gear is gorgeous, as is the weapon, but there's something about the classic sword and board type knight, especially in some regal armour that looks awesome. I'd really like a cloak vanity slot though and to actually do some pvp for the lionsmane gear.

Overall, I think once I have it leveled to 60 I'll be going PLD for everything except dungeon farming where the extra dps of DRK will really come in handy.