One important difference between Shielding in MBAC and Parrying in 3rd Strike are the available options the Shielded/Parried opponent has in his situation.

Shielding: Your attack completes its motion, and cannot be cancelled into another attack unless it is whiff-cancellable. However, A attacks can be whiff-cancelled into Shields.

Parrying: Your attack may complete its motion, but counts as a 'hit' and can therefore be cancelled into another attack.

You would think that Shielding is more powerful from a defensive standpoint, because it commits your opponent into completing his move. However, there's something else to consider, as the defender.

Shielding: After [EX]Shielding, you can cancel into another Shield, or an attack/command. However, you may not block. Your character is stuck in Shield recovery frames.

Parrying: After a Parry, you are allowed to parry again, move, attack, or block. Your recovery frames simply reset, with full freedom afterwards.

Please keep this in mind when attempting to Shield attacks. You MUST complete the Shield, or attempt an interrupt. Therefore, they're actually riskier than parries (not withstanding the miss animation). However, if you can read the animation frames of your opponent, it pays off even better than a Parry would.

Game Mechanics: Discussion Thread