Aeonk is not entirely wrong, iLvl 115 NQ gear is actually 'weaker' than unique (blue) iLvl 100 gear!

Take a look at the head gear:
Weathered Noct Circlet
Mythrite Sallet of Fending

Now you might say, 'The Mythrite Sallet has a few more strength and vitality points', and yes, that is true...

BUT! take a look at those defense values. The Mythrite Sallet has almost 10% less defense than the Weathered Noct Circlet.
And that's where incoming damage starts to hurt.

From the healer's site, I would prefer healing a tank with higher defense values than one with a higher HP pool.

Additionally, afaik before you reach Ishgard for the first time (50+ after MSQ which takes years), you can't buy iLvl 115 gear from vendor (I might be wrong tho).

@OP: You might or might not be able to get your whole ironworks gear ready before Ishgard, depending on how many dailies and dungeon side quests you complete.

Always do your job quest as soon as possible. Your reward is an additional skill/trait/ability and at 50, 60, and 70 new gear. This gear itself is good enough for most dungeons and trials, even through it may put a greater burden on your healer - better let another tank be MT (switch to Sword Oath; use Shirk on MT if you get close to aggro) - as long as it's not a newcomer, too.

Only crucial point is accessories, which you will get by MSQ and poetics only. They give a good bunch of HP or Str, depending on what you can effort to use. But they don't increase your defense, so they aren't your priority.