In terms of raw numbers? Dark Knight wins by a landslide. Their DPS rotation has a higher average potency-per-GCD than PLD does, and their "DPS stance" (not really a stance, so much as a lack of tanking stance) gives them a pretty significant boost to their overall output. They're not going to top WAR in Deliverance, but neither can some DPS classes.

In the MT slot, DRK actually has the potential to beat out WAR for DPS, if it's played right, while also providing near-PLD levels of damage mitigation.

If you bring PLD as MT, you do so because they have the greatest capacity to mitigate spike damage and tankbusters, and not for their DPS output. If you bring them as OT, you do so because of their spot-healing ability with Clemency, Cover (which can help cheese certain mechanics and swaps), and Divine Veil (a group-wide 2500 HP shield can be extremely helpful), and not for their DPS output.