House-and-land is a single contract with a single party who supplies you both the section and the dwelling. The price you sign at is meant to be the price you pay, the section's already developed, and the timeline is closer to the optimistic end of feasible. The trade-offs are: the floor plan options are usually a fixed plan library, the section was chosen by the developer (not you), and you're tied to that builder's subcontractor list.
Build-on-section means you've bought a piece of land separately and you're contracting a builder to construct on it. You get more freedom — over the section, the design, sometimes the materials — and you carry more risk. Section preparation costs (driveway, services, retaining, geotech surprises) are yours, not the builder's. Timelines are usually longer because everything is bespoke rather than off-the-shelf.
The honest middle ground that most buyers end up at: house-and-land for first homes and second homes where the priority is certainty; build-on-section for the third-or-fourth-home buyer who already owns the land or has strong views about what they want.
What to verify in either case: whose name is on the section title at each stage of the build. Some house-and-land arrangements settle the section to you on signing; others not until practical completion. The cashflow implications are not small.