Determination increases your damage by ~50% more than tenacity does.
Tenacity is good at what it does: increasing damage dealt while reducing damage taken (and healing increased by your OWN actions, but not from other sources)
Technically, you want to avoid tenacity for det, crit and a comfortable SkS. Nevertheless you WILL have tenacity on your gear, and because stat increase works in tiers, melding a little tenacity CAN indeed increase your damage more than melding another det materia.
e.g. if you are at 1220 ten, and get the next increase in <=60 pts (on Lvl 80 gear), but you wouldn't get the next increase in det in <=60
-thus wasted materia meld-, it's better to meld tenacity.
For exact numbers check this sheet
FFXIV LV80 Stat Tiers by Gouka Mekkyaku
or this website
Theoryjerks.
FYI, you want to have at least 1523 SkS on WAR for 2.38s GCD, some suggest 2.37s at 1624+ or even 2.36s at 1726+ if you need breathing room. Any faster than that is basically meme and/or wasted.
edit: I feel most comfortable with 2.37s GCD as it aligns with IR the best.
Don't even bother melding DH, as already mentioned it's better to meld determination or tenacity if you get to the next tier. Your DHCrit actions (IR and IC) do about
45% of your overall dmg,
maybe a little more or less -compared to SB IR did 30% of your overall dmg- so even ten increases your DPS more than DH does.
If you go for omni-tank don't bother with DH neither.
You get 0.1% (expected) damage increase every 25.33~ det pts, or 24 DH pts. The loss is marginal on other tanks in comparison to WAR.