Confused why you're doing maths on NIN. Surely the NIN will use Shadewalker regardless of who pulls, thus resulting in a flat, equal enmity gain on both sides.
The argument in favor of WAR MT opener is that the DRK will never, ever need to use PS (among other factors). Accounting for only 1 PS in an entire encounter is realistically only going to work in something like A6S or A8S phase 1, because generally people will get a lot of uptime on the boss and therefore produce a lot of enmity.
Another thing to consider is that when a WAR pulls they're also going to use Vengeance with Berserk, IR, Bloodbath, etc. up because you're realistically not going to need Vengeance ever within 2 minutes of the start of a boss encounter - you have RI and Thrill + Convo available to you if you need a cooldown. I mention Bloodbath, because of my next point which is...
Healer DPS. By having a WAR pull, not only will you get tank stance's default "mitigation", you also get the 30% damage reduction which will presumably lead to more GCDs for both healers to use before needing to actually heal.
Basically, with DRK MT you gain 1 FC in place of a Maim (which you forgot in your WAR MT opener or maybe you were trying to use equal GCDs between both MT openers idk), lose Vengeance ticks vs 1 Reprisal + Low Blow reset, lose healer DPS, and also a bunch of enmity that will probably be needed on any encounter that doesn't disappear/die really quickly. Reprisals/Low Blow resets also will still actually exist since you'll swap after the WAR MT opener, but in the DRK MT opener the DRK just keeps MTing so you entirely lose the Vengeance ticks.
Anyway, it's really min/max and now that Gordias Savage has happened you probably won't need to min/max hard enough to meet any DPS checks. Just do whichever your group is comfortable with.
e: One more thing, encounters aren't all striking dummies that can have set in stone openers for every single one. Some times it's better for the DRK to pull, sometimes it's better for the WAR to pull. It really depends on what damage goes out at the start of the fight and who needs to end up tanking what when. Experiment and see what gives you the best results.