Originally posted by Midas:

Iron buckler is just worse than the iron shield, though, even for parrying.

That is not true. With a fully upgraded Iron Buckler you can block for 40 and parry up to 100 damage. While a fully upgraded Banded Shield can block for 54, but will only parry up to 81 damage. The reason for that is the parry bonus multiplier, which is 2.5x on Bucklers and only 1.5x on Round Shields.

The advantages of Bucklers or Round Shields are obvious. Take the Buckler if you plan and are able to parry frequently. Take the Banded Shield, if you want the security of a higher raw block, if you tend to mess up the timing for a parry sometimes.