PLD is the tankiest tank in the early game if you are MSQ locked.
DRK is the most complete tank early if you have already reached Heavensward on a different class. You start at lv30.

AoE damage, very important when pulling larger packs:
PLD - Total Ecplise lv46, and Circle of Scorn at lv50
DRK - Unleash lv6, Dark Passenger lv40, and Salted Earth lv52

Single DPS target rotation:
PLD - Fast Blade, Riot Blade, Goring Blade lv54 / Riot Blade lv60
DRK - Hard Slash, Siphon Strike and Soul Eater lv38

In all situations, DRK gets more of its toolkit soon than the PLD.