SMPTE Timecode is for frame sync: https://resolume.com/support/en/smpte
Ableton link is for sharing the beatmark position, so all players stay in beat sync https://resolume.com/support/en/link
SMPTE will have a clip play at the exact frame as the timecode (+offset). A SMPTE synced clip will only show if the input timecode position is in the range of the clip.
Ableton link will only give the beat mark and tempo, so you can launch beat synced (if enabled), but have the freedom of launching a bar later if you want to.
Does this make the difference a bit clearer?