Dodge vs Shield Block Rating... this feels like new territory to explore and mutilate, however it's really just the old "Avoidance vs Mitigation" debate.
Dodge gives no significant bonus other than avoiding an otherwise-damaging incoming blow. Granted, this alone is benefit enough to merit using Dodge, but I believe it is only useful to a certain extent. Tanks are fueled by Rage (speaking only from the Warrior's perspective here), and Dodging a blow doesn't confer any rage-gaining benefits. (I personally feel that this should change, but I digress...)
Shield Block Rating appears to be not-as-useful, but I consider it akin to a side-grade: useful in the same application, but in a different way. With Improved Shield Block, you're gaining at least 1 rage from each block, plus the rage from the damage dealt, thus enabling you to generate more threat in a shorter amount of time. Personally, I'm a fan of the "Crush Immune" ideologies to push the Block% to its max. If you're blocking more than you're dodging/parrying, you're cutting off a sizable chunk of damage off each blow. (This effect is magnified in PvP, where you have dual-wielders that quickly chip away at your life... you'd be surprised how often I saw "Block" (i.e., no damage dealt) with a Block Value around 530. I laughed everytime I got sap'd out of the blue cause I knew how the following 30 seconds or so were going to go.
Ultimately, the difference between the two varies upon preference and your specific build. If you're building for threat-generation and mitigation, spec for Shield Block Rating. If you prefer avoidance and relying on chance to see you through, feel free to spec into Dodge... but don't expect to be invited into many end-game raids. Those "streaks of bad luck" tend to happen often and always at the worst times.
Shield Spikes FTW!
