The short verdict
Choose Shopify Plus if you want your team working on merchandising, marketing and customers instead of servers, patches and upgrade projects. It covers B2B, international selling and checkout customization natively, and custom logic runs as apps and Shopify Functions outside the platform code.
Choose Magento (Magento Open Source or Adobe Commerce) if you truly need to change platform behavior in PHP, must control where and how the application is hosted, or depend on Adobe Commerce B2B workflows such as negotiable quotes, purchase order approvals and requisition lists (Adobe docs) and have the developers to maintain them.
If you are still on Magento 1, the decision is already made for you: Magento 1 reached end of life on June 30, 2020, with no security patches from Adobe after that date (Help Net Security). You need a new platform either way.