Vengence: 120s / Shadow Wall: 180s / Sentinal: 180s
Inner Beast: 60 seconds (using infuriate to reach 10s uptime) / Shadowskin: 90s / Rampart: 90s
Raw Intuition:90s / Dark Dance: 60s / Bulwark 180s
Their most common mitigation, and most likely the one to be most frequently in rotation, rampart, is on-par with Shadowskin, and a little less available than Inner Beast. However, Inner Beast is GREAT for on-demand mitigation for instants.
Let's take A1S as an example.
Warrior can do: Inner Beast #1 on missile cast, Inner Beast #2 before the Hypercompressed Plasma, and have Vengence running the entire time (30% uptime entire time, two spikes of 20% mitigation. Could argue that's nearly 50% for the duration, but requires tank stance to do it.)
Dark Knight: Dark Mind for tank buster, Shadowskin for the duration (Effectively the same as WAR.) Has Shadow Wall as well.
Paladin: Sentinel on the first/third, Rampart for the duration. Arguably the situation you mentioned - where they would hallow the second.
Looking it over, I get what you're saying - PLD wont' be garunteed the same level of mitigation consistently in the rotation for magic damage. As stated, however, that partially arises from the issue of Physical vs. Magical. Dark Knight would suffer the same issue on the inverse - if the tank buster wasn't Magic-Based, they'd consistently take more damage than Paladin to the same tank buster and the Paladin could just alternate in their Bulwark to cover the Hallowed.
Let's not neglect that, in physical encounters, Paladins have an additional level of mitigation on top of their already relatively equal mitigation. As stated, they also have Stoneskin, Clemency, and Divine Veil to help with keeping themselves alive (Can't actually recall if divine veil procs on themselves though.)
Tl;dr: Duration wise, they're pretty close and equivelent, with Paladin lagging just a little bit behind... for magical encounters.
Edit: And of course, ym original point...