Well, like what was said, everyone has the same shield in the end so it doesnt matter. But before that, i'm pretty convinced there's only one way to go.
Blockrate is based on your DEX, and block strength is based on your STR. So any number you'll get is going to deviate between players due to diffirent stats. The performance of your shield is minimal at best. It doesnt really matter.
At best, the diffirence between the lowest and highest non-relic amount of blockstrength (at lv.50) is about a 11% reduction (with a 3 point Blockstrength Lv.1 shield) vs. a ~22% reduction. That's because your innate attributes weight in a whole lot more than the block-strength/rate on your shield.
Blockrate doesnt offer such convenient text lines with percentages in the damage log. But take it from me, it's crappy regardless of the amounts. I'm convinced the smaller, high block-rate, shields can get a good 20% blockrate on them. But this is eyeballing from the amount of Shield Swipes they offer. I've switched between the Holy Shield +1 (with 181 Blockrate) and that Lv.1 Square Maple Shield (with 14 Blockrate), and i dont really see much of a diffirence in blockrate. I dont carry any heavier types on me for testing (for obvious reasons), but a tower shield of any kind is not going to give you much extra blockrate over your base values from your DEX attribute.
tl;dr: Blockstrength is such a trivial thing, it's not worth gearing towards. Stick to whatever gives you the highest blockrate if you can regardless of how much blockstrength it has. No matter what you pick, it's not going to matter much, in the end it balances out to that all the shields are roughly same for you, except for Holy Shield and it's +1. Which are superior no matter what.
Basicly, do you want to block 10% (number pulled completely out of my ***) of the attacks for a 20% reduction, or do you want to block 20% of the attacks for a 10% reduction. But atleast with more blockrate you get more Shield Swipe procs.
Edited, Nov 5th 2013 3:10pm by KojiroSoma