It depends on the content.

With Twintania, one of the biggest threats is DS. You want to block it, so more block rate be better yo.

With Turn 6 and Raffy, her attacks are general consistent. Bloody is nowhere near as strong as deathsentence and even more predictable. She does however have a pretty strong auto attack that can crit (A use for awareness, finally) but tank burst damage is not a significant factor on this fight. Therefore, a more average shield would be better. This is under the assumption that like the onion shield, a higher block rate shield has less overall mitigation then a kite shield.

For the Block Strength shield. Well, I don't know. We don't really have math on how it performs in theory, or any experience with its practical application in endgame content.

TLDR: Fight with high burst sections where you need to block more. High rate. Otherwise the shield with the best overall mitigation or stat weights.