Paladin... Shield...block or parry?

[VisRalis]

High parry shields are good for all purposes. It has a high rate to parry incoming attacks to reduce damage taken.
But parry shields usually have lower Block (damage reduction), so while you can frequently parry attacks, it doesn't block as much damage.
High block shields are the reverse - doesn't parry often but when they do, it reduces significantly more damage.

High block/low parry rate shields are good when you use your skill Bulwark to increase block rate (+60% 15secs) to take on a big hit/nuke from a Boss.
Cooldown on Bulwark is 180secs so you'll find yourself mainly using it for big pulls or saving it for known boss attacks.

[Vandril]

High block/low parry rate shields are good when you use your skill Bulwark to increase block rate (+60% 15secs) to take on a big hit/nuke from a Boss.
Cooldown on Bulwark is 180secs so you'll find yourself mainly using it for big pulls or saving it for known boss attacks.

Lest I'm mistaken, that's a +60% increase to your current block rate, not a straight +60% chance to block. Bulwark will therefore have greater effect on shields with high parry. Due to this, it's not as black-and-white as it at first seems.

There tend to be three types of shields in the game (labeled as types 1-3 for convenience): high parry, low block (Type 1); low parry, high block (Type 2); moderate parry, moderate block (Type 3). If I recall correctly and if the stat weights haven't changed since 2.0, I believe the math was done showing that Type 3 > Type 1 > Type 2, with the difference honestly being negligible.

Urthdigger has the right of it in that it's the item level that matters. Almost always, choose the shield with the higher item level. When item levels are the same, I would choose a shield based on the "type rank" I mentioned above.

Now, personally, I dislike the low parry shields. Sure, they block well when a block occurs, but they also don't block often. This means that your damage taken becomes more wild and unpredictable, which is generally considered a bad thing as a tank. High parry shields fix this by parrying often but blocking less damage, causing a more smooth and predictable (and thus easier to heal) damage intake. The moderate shields tend to have the best of both worlds - enough parry for a fair lot of blocking with enough block for a fair lot of mitigation.