Good morning, thanks for giving me something to read when I woke up.
If you guys think about other games then you'll realize that you really do need to not only have a specific class tank in specific fights, but even within that class they often need to re-spec for each dungeon in order to overcome specific dungeon mechanics. For the endgame content I do think you'll want full heavy for full mitigation, not necessarily because of the Armor stat, but because the set bonuses for heavy will be geared for tanking rather than damage and such on medium.
You can calculate it out right now, but it'll change when we know what the hard numbers on enchantments are. I do agree that medium armor would allow you to keep evasion/immovable up a bit more and that might give you more mitigation in the long run, but with regen enchantments it might not even be necessary.
I do think you'll want at least 1 piece of Medium for Evasion though, in which case I'd take 2 that give a nice 2-piece set bonus. I am almost certain that if I'm tanking I'll be using 5h+2m because Evasion is really strong, and taking 1 piece of something when I could get a bonus for taking another where a bonus most likely won't fit in with 6 heavy is probably going to be better than just 6h/1m.
DK is the logical choice for damage mitigation. But, for overcoming mechanics (which has always been the most important factor in tanking) then DK doesn't necessarily have better options than say a Sorc who can bolt escape (Puncture then Bolt Escape, repeat until the DPS kill the target. This could be a common necessity.) and then maybe winged twilight will be a nice addition to tanking through a morph. (Seems likely, since Twilight was initially supposed to be tanky.)
In another situation it might be better for a NB to stand away from the group while tanking, Puncture and then Cloak. The target would then run toward the rest of the group for 2.5s and then the NB would reappear and the target would run back to the NB for 2.5s (since it's a 5s taunt), meaning that it wouldn't be able to do damage in melee range anyway for pretty much 5 seconds. Like we kind of said before, I don't think that most interrupts, slows, and incapacitates will work on the bigger boss-fights..so that's something to take into account when weighing mitigation.
But, we have no idea what mechanics the dungeons will bring us.. this is why I only really care about PVP theorycraft at this point, because that's the only thing we have an idea of what the enemy will be like.
In PVP I think that you're right on, irons, that you'll have plenty of mitigation and you'd do better overall by getting more damage and utility as a DK tank in PVP.