If you're going with DRK, I'd say have at it. It's a very simple tank. Too simple, but that's a good thing for most so I won't rag on it. The class has definitely seen better expansions, hell I'd even take the 4.x iteration. However, that's not to say it's a bad class. It's good, honestly. It has all the tools every tank needs: some reliable defenses, and a decent damage output, but it's become so brain-dead to play. Press 123, and cycle some oGCDs in between whenever the need arises. I guess Squeenix has been pushing too hard to appease casual players? Probably. Or the department in charge of class design has no idea how to make a DRK not... well, whatever it's become these days.

I play it because it I like to play sword-wielding classes in MMOs. I adore my DRK and don't regret playing it. I have a WAR, but I just cannot be bothered to play it right now. Never tried GNB, and I never got PLD past it's youthful self (GLD). I suppose the best advice is to play them all in order to get a better feel for them. But if you're pressed for time, go with the one you think looks cooler. In the end, the true endgame is vanity. It's not having the best gear in terms of function, it's having the best gear in terms of fashion.