Server-side
Nothing to install, works from every device including phones, and nobody can bypass it. The trade-off: senders do not see the signature while writing, and the copy in Sent Items may not show it.
Client-side
The signature appears in the compose window, which people prefer, and the sent copy matches what the recipient sees. It relies on the mail client syncing, so it is best paired with server-side enforcement.
Recommendation
Use client-side signatures for everyday visibility and server-side stamping for legal disclaimers that must never be missing.