Ads stop the moment you stop paying. SEO compounds. But ads bring traffic today while SEO takes months. The answer: both. But not at the same time.
Start With SEO. Layer Ads Later.
Month 1-6: SEO only. Build content. Get indexed. Start ranking. Month 7+: add PPC for keywords you are not yet ranking for. Use PPC data to find which keywords convert best — then write SEO content for those.