MT/OT is an archaic notation, and didn't exist in its original meaning after ARR. You're probably better off calling them T1 and T2.

If you made your two tanks choose between one of them gaining a damage buff and the other a mitigation buff, I can predict what the overwhelming majority is going to be fighting over. It's bad enough when there's forced downtime on a mechanic which affects one tank but not both, because you'll have disagreements over who is forced off the boss.

You can't trade off mitigation against damage output. Mitigation is a pass-fail check. All tanks need to be able to meet the mitigation checks of every encounter, or else they get excluded from content. Anything over that, however, is superfluous. These two need to be independently balanced on all tanks.

As far as dps is concerned, all tanks should have dps parity. People come up with all kinds of clever excuses around why their job 'deserves' to have advantages over everyone else, but it creates boring comps where everyone runs the same jobs for years at a time. We've had this for most of the game's life, and it's a serious design problem.

As far as mitigation, self-healing, and utility effects are concerned, the core problem is partial-homogenization. Complete homogenization of an action can be a bit stale, but it's fine from a balance perspective. Nobody can find a balance problem in Heart of Light and Dark Missionary being identical. The problem is when you have two abilities that you can directly compare, and one is clearly superior to the others. The classic historical example was Vengeance vs. Shadow Wall, when Shadow Wall provided the same effect with a 60 second longer recast (and why does Vengeance still have a thorns effect not present on any of the other 30% DR cooldowns?) You see similar problems with raid mitigation, where PLD and WAR's raidwide mitigation tools are simply superior to DRK and GNB. And that's before we even start to talk about discrepancies in self-healing across all the tanks.

Bottom line, all of these need to be balanced simultaneously.