Ochain 90
* 40 DEF
* no shield skill
* 60% damage reduction on block
* 108% proc rate with skill
* Converts 25% of blocked damage to MP
* Net DT reduction = 60%
Duban stage 2
* 145 DEF
* 125 shield skill
* 60% damage reduction on block
* ~138% proc rate with skill
* 15% MDTII
* 25 MEva
* Net DT reduction = 60%
Srivatsa
* 150 DEF / HP / MP
* 129 Shield Skill
* 75% damage reduction on block
* 80% proc rate with skill
* 15 Enmity
* 8% DT
* 5% Annuls damage
* Net DT reduction = 1 - (1 - 0.75 * 0.8) * (1 - 0.05) = 62%
The true proc rate varies based on your attacker's combat skill, which is why it's valuable to have a nominally >100% proc rate. It's also nice to have 100% of hits be blocked for consistency reasons (no spikes) and to reduce interruptions, although people tend to use SIRD sets now. Also, Shield Defense Bonus (maybe 2 damage off each hit) applies to any block equally. As soon as Srivatsa's proc rate drops below 77% (any real content) it has less net DT than the other two shields, although 5% of its damage reduction is pretty unique and works when you're turned around.
You can see that Duban stage 2 blows the other options out of the water in any endgame tanking situation. Aegis is better than it for magic damage.