Defender is a piercing shield. It's rather cheap to get considering missions give you the recipes, but it's usually outclassed by Barbarous Thorn Shield. Piercing damage itself isn't really something to worry about, shadow and elemental are more dangerous.

Plate Shield protects only against normal, unless you craft it into an Ironmight Plate Shield. Volcanic Plate Shield is a great shield for King of Ashes, because of the fire resist. It easily tanks Vanaduke's mace. However, it'll have difficulty everywhere else, since it only protects against normal damage.

Owlite protects against elemental damage. Elemental and Shadow are far more dangerous than Piercing. Owlite is a nice shield since it makes dealing with the gun puppies much easier. Owlite has very good status resists (Shock and Fire), and is also a very good shield for King of Ashes.

Ideally though, you should have a shield for each damage type. A shield for elemental, a shield for shadow, and a shield for piercing. However, a lot of people (including me) tend to stick with one shield until they reach 5*. Defender is kinda "meh". You have to turn the plate shield into 5* before it has its fire resist from Volcanic Plate, and it only protects against normal. I recommend Owlite.