SEO comparison, does either platform rank better?

Both can rank well. Different shapes of problem:

Shopify SEO strengths: fast defaults, automatic image lazy-loading, built-in sitemap, clean URL structure, automatic structured-data (Product, BreadcrumbList) on most themes. Weaknesses: forced /collections/ and /products/ URL prefixes, limited control over canonicalization, no native hreflang for multi-region (Shopify Markets sets it, but customization is awkward).

Magento SEO strengths: full control over URL structure, robust hreflang via store views or custom modules, custom canonical rules, JSON-LD structured data per page (with the right modules), built-in XML sitemap with priority/changefreq controls. Weaknesses: defaults need tuning (duplicate-content from layered nav, slow LCP on Luma), out-of-box, Magento ranks worse than Shopify on technical SEO until you fix it.

In practice: a well-engineered Magento store outranks Shopify because you have more levers (URL structure, canonicalization, structured data, programmatic SEO at scale via category landing pages). A poorly-engineered Magento store loses to a default Shopify store. SEO advantage goes to whichever team puts in the work, neither platform wins this fight by default.