1) Shield Bash is a GCD skill while Low Blow is not.

This means that Low Blow can be used at any time and does not interrupt your combos.

However, Shield Bash doesn't have a cooldown so you can use it more frequently, such as to stun multiple targets during some trash pulls.

Typically, Low Blow is the superior option because it's more easily used. That, given with the infrequent need to stun targets, mitigates some of Shield Bash's benefits. Though, it's useful to keep both in mind depending on what content you're running and if some of Shield Bash's pros outweigh Low Blow.

2) Silence does not have diminishing returns. Meaning you can keep silencing targets repeatedly and they will never become immune. Meanwhile, stuns are affected by DR so their durations are reduced by 50% twice and then the target becomes immune until 1 minute after the last stun they were affected by. During that time, a stun will do nothing. As a result, this is why Silences tend to be quite short duration, they're pretty much just used as interrupts for players than actual silence effects.