Carapace Shield: Regular block with level one shield and 0 skill in blocking negates 96 Damage. If you manage to parry it negates 144 damage (block armor x parry bonus 1,5)

Carapace Buckler: regular block with level one buckler and 0 blocking skill negates 78 damage. If you manage to parry it negates 195 damage (block armor x parry bonus 2,5)

A level 2 buckler with succesful parry will negate 210 damage, which is the highest possible amount rn in the game, compared to a level 2 shield wich will parry 153 damage.

Personally i go with the shield since it's more reliable when fighting multiple enemies.
But really the choice is up to you and your preferred playstyle.