Scope
Websites contain navigation, multiple topics and many user journeys. Landing pages intentionally narrow the experience.
Lifecycle
A website is usually persistent. Landing pages can be campaign-specific, seasonal, experimental or tied to a single offer.
Measurement
Landing pages make campaign measurement cleaner because they reduce unrelated paths. Website pages often require more segmented analysis.
You usually need both
Most businesses benefit from a durable website plus dedicated landing pages for campaigns that need stronger message match and conversion focus.