Contentful and Sanity are both headless CMS platforms for developers. Contentful is known for an established, enterprise-grade platform with a large integration ecosystem. Sanity is known for real-time collaborative editing (Sanity Studio) and highly customizable, open-source content modeling. Neither is objectively "better." The right pick depends on what you're optimizing for.
Choose Contentful if you need enterprise scale and established third-party integrations. Choose Sanity if you want a fully customizable, self-hostable studio with real-time collaboration.