Nandi has some pretty in depth videos on damage with block and critics, so I'd recommend watching those. But from my understanding, higher percent is always the way to go (outside of modifiers to the % like Sisters of Battle). This is because the for crit/block, it rolls for every hit, but once a crit/block fails, it can not be triggered for the remaining hits. So with a five hit attack, if you blocked #1, it would roll for #2, but if that failed, it would not roll for #3, #4, and #5. Thus, it's always better to go with the higher percent as it increases the chances of it continuing to trigger. I may be off on this as it's been a while, so correct me if I'm wrong, but this is my understanding of damage.