Shopify and Magento are very different platforms when it comes to backend usability and by this I mean how the store owner manages products, orders, customers and not how the customer uses the site.
Shopify's backend is developed by Shopify and has very little to no flexibility (apart from through Apps). Magento's backend is developed by Magento, however can be completely customised by your development partner to meet your exact needs. With that flexibility comes complexity, which isn't necessarily a bad thing, but it does depend what you're looking for.
When brands approach us and are considering changing platforms, part of our discovery process is understanding their business process. In a perfect world, how would they manage products, customers, orders and other backend functions? If flexibility of how the orders screen looks, or how orders should be dispatched are absolutely essential, Magento tends to be the platform we lean towards. If simplicity is the goal, and there's no custom requirements, Shopify starts to look like a great option.
The other important consideration is the team structure and how the client plans to manage their website moving forward. Is there a larger eCommerce team in place, an online manager or is the owner going to manage the store themselves?
Magento is a big platform, and as a result typically requires a larger team in place to effectively manage and grow using it. We have clients that have teams of people managing different elements of the site to ensure everything from visual merchandising to customer service is taken care of. Magento's documentation is very high level and doesn't often relate to the actual functions and process of the individual business so internal training is key.