Mostly it depends on your other defenses--a blocking item is a lot less useful if you're already dodging from 4 or 5, for instance, while it's crucial if your defense score is otherwise 1 or 2.
So the question that needs asking is do you need to enhance your defense roll, or replace it? (Or do what one of my players does, and construct a 6-shift block alongside an Armor:2 item to compensate for the character's Average Athletics score).
Also, this sort of question should go in the main DFRPG section, not the Resource Collection.