If you are validating an offer or running paid campaigns, a focused landing page is usually the fastest way to test demand. It reduces build time and keeps the message aligned with one traffic source.
A full website becomes the better move when you have multiple offers, need stronger trust pages, or want to support organic search over time.
The right sequence for many teams is simple: start with a high-performing landing page, then expand into a full site once message-market fit is clearer.