in the end - the buckler is superior when facing against one or two creatures AND you're a boss at timing the parry (while facing them).
The shield is superior against a larger swatch of enemies because continuous holding block absorbs more dmg; and is still quite effective if/when you time the parry.
Also I think the shields have higher "block power" which I think stuns them longer if parried successfully - but honestly it's nominal effect in the end as I usually kill most creatures shortly after parrying.

If your playstyle includes a lot of parkour and taking advantage of terrain to limit the number of creatures that can engage you at one time, the buckler is the way to go.
If your playstyle is more about blocking and staying protected until you get your stamina up and can make good attacks, the shield is the way to go.

My playstyle is more the first but I choose to go shield because i am nearly always with my wife and/or our two close friends and I tend to do the shield obstruct while she pelts with arrows. But when by myself, I excel with the buckler.