When he talked about the block strength vs. block rate, what he meant was that there will rarely be a situation where you have two shields with the same item level with differing block rate and block strength. Thus, since item level trumps all, the benefits of one over the other is irrelevant more often then not, since you'll rarely have the position to choose with all else being held equal. However, if you are given such a choice, it is up to personal preference.