The core difference
With SEM you pay per click and placement stops the moment the budget stops. With SEO you invest in pages and authority, and traffic continues after spend ends - but it takes months to arrive and cannot be switched on for a launch date.
Speed versus durability
Paid search delivers data in days: which keywords convert, which landing page copy works, what a lead actually costs. Organic delivers durability: a page that ranks for three years costs nothing extra on the ten-thousandth visit. Mature teams use paid as the research budget for organic.
Cost structure
SEM cost scales linearly with volume; SEO cost is mostly fixed and front-loaded. In expensive categories - insurance, legal, B2B software - where clicks run into double digits, the payback case for organic is strongest.
Running them together
Export converting search terms from your ads account and build organic pages for the top ones. Conversely, run ads on organic pages that rank on page two to test whether the intent converts before investing more content effort there.