1st thing is there is a difference between blocking and parrying.
Blocking is just blocking.
Parrying is when you block slightly before the enemy attack comes in stumbling the enemy and inceasing it's recived damage by 2x for a short time period. (yes this also stumbles archers)

The Buckler is worse for Blocking but better for parrying because it has a 2.5x parry bonus instead of 1.5 for the round shield.