Materia with real benefits suggestion

[tdb]

I believe it was worked out that melding for 2500 Determination gives you 10% extra damage, while 2500 Tenacity gives you 7.5% extra damage and defence.
That's 2.5% extra damage, and considering you will never have ZERO Determination, it's probably more like 1.5% extra damage.
Meanwhile a conservative 5% extra defence from Tenacity... is basically an extra cooldown. Given that most of them are ~10% damage reduction with <50% uptime, a 5% constant is fairly useful for things like dungeon trash pulls.

The real tradeoff here is whether the extra defence from tenacity will allow the healer to get off some extra DPS. If a WHM can skip an Afflatus Solace and cast a Holy instead that's easily worth the 1.5% DPS loss for the tank.

Piety is in a similar boat, in that you either need that extra MP, or you don't.

My beef with Piety is that different healers seem to have different needs for it. WHM is fine with zero piety melds, but AST seems to run out of MP every now and then. Though admittedly my AST isn't at Lv80 yet and class balance before max level is always a bit so-so. And I should probably learn to use Lightspeed more.

[Seraphor]

The real tradeoff here is whether the extra defence from tenacity will allow the healer to get off some extra DPS. If a WHM can skip an Afflatus Solace and cast a Holy instead that's easily worth the 1.5% DPS loss for the tank.

Not really a concern outside of Savage/Ultimate.
In casual content, Healers likely aren't getting the most dps spells out anyway, raising more than they should be ideally, healing more because dps are taking more damage, and hesitating during mechanics to work out what to do next. (remember, all of these calculations only pay off at peak player performance, which is 5% of the player base or less)
Meanwhile, tanks are popping all of their cooldowns on dungeon trash pulls, and then dying because the healer can't keep up once they've all worn off.
Player skill aside, you'd probably die more as a tank for this reason than any other. So that's when this extra 'tenacity cooldown' pays off.
Therefore, if you're not tanking in Savage/Ultimate, you probably get more use out of Tenacity than Determination. Even if you're doing the latest Extremes as your highest level content as a tank, unless you're consistently failing enrage at 0.2%, you'd still get more use out of Tenacity for all of those dungeon runs you do in between.