They should likely be considered equivalent (that is, the Cloak of Peaceful Resolutions should have an effective iLevel of 200, though not necessarily considered purple). This item is best in slot for tanks until Naxx 25 (or 25 valor emblems), when the upgrade is iLevel 213. In the interim, even tanks who have Cloak of Armed Strife are taking a rating hit by choosing to use the better (blue) cloak.
The Seal of the Pantheon, a tanking trinket from normal Loken, is best in slot for many tanks, once again until Naxx 25. It's technically blue and iLevel 187, but it should be set as effectively iLevel 200 with its ridiculously high +def rating (+13 def skill! Almost 10% needed for def "cap") on a trinket. Since there are no superior trinkets at iLevel 200 (the upgrade is from Naxx 25), this should be considered 200.
And finally, the Essence of Gossamer really remains best in trinket slot for almost all tanks, with a whopping +111 Stamina on a trinket and a really great proc. iLevel=200 is probably fine, but considering this effectively an epic item would better weight the rating to reflect its crazy usefulness.
Keep in mind that Maexxna's cloak has extra armor in it, which is a nice mitigation stat. If you do not want extra armor, you can take Gale-Proof Cloak from Sartharion-10. If anything, I would downrate the three cloaks with extra armor but not uprate the normal cloaks.
When I compare two of my characters (tank and DD) that are nearly equally equipped, I can see that the Essence of Gossamer still seems to be underrated. Both characters wear 9 items of ilvl 213 and 8 of ilvl 200, while the tank having two of them blue (one of them is the Essence of Gossamer).
The pure gear score difference without enchants is 49. I guess that is very much for the both characters having nearly the same level of equipment.
You're entirely wrong about the cloaks. Armor is a very valuable stat. The cloak you mentioned is decent, and actually over budget. Compare it to Cloak of the Enemy from H Nexus.
Gossamer is inferior to the Valor Medal, Repelling Charge, and any Ulduar tank trinket. Amusingly, both ilevel 200 tank trinkets are better than both ilevel 213 tank trinkets. Gossamer is overrated because a big jump in stamina looks really good on your health bar; the avoidance from other trinkets is superior. Also its proc is utterly worthless.
Seal of the Pantheon is a decent trinket, but it's not as good as you're saying. The only reason to use it in late raiding is as a stopgap measure to maintain 540 defense after a particularly awkward series of upgrades.
