I need to get some sleep at the moment. My very, very quick response which I'll probably respond to when I roll out of bed is, the point at which I'm making decisions between these two enchants, I probably ought to just pick up a second shield and enchant them different ways.
18 Block Value is essentially better when your Effective Health is way beyond the fight. If you can go 5+ seconds without a heal and live, even if just barely, 18 Block Value probably wins. If your in a 5-man or Heroic 5-man, it definitely wins.
ok, sleep, but yeah, Stamina is what is best in most situations, but Block Value has it's time and place.