PCI compliance, Shopify hosted vs Magento self-hosted?

Shopify publishes PCI information for its service. A provider assessment does not by itself determine every obligation for your business or custom checkout. For a self-hosted Magento store, the payment flow, hosting, application and integrations also affect what must be assessed. Confirm the relevant provider evidence and your responsibility allocation with your payment provider, acquirer or qualified assessor. A platform comparison cannot assign a particular self-assessment questionnaire or guarantee merchant compliance.

Official references: Shopify PCI information; PCI SSC outsourcing responsibilities.