I think there isn't a right or wrong - I've healed STR DRKs who knew how to tank and did fine pulling more than 2 sets in Neverreap. I've also healed VIT DRKs who never used grit or darkside and almost die to a pack of 3 while never using cooldowns. I think the main thing is experience -- if you have experience being a good tank, it doesn't matter what you wear // at that point it is personal preference for however you like to play. If you're a new tank, I highly recommend VIT until you're at least proficient enough to know how and when to swap to a dps stance and maximize STR gear. The problem is people new to tanking trying to tank like those who have been tanking FF14 for years, and trying to gear like those experienced tanks, as well.