Quill can be used for the get going quickly drop in use case. Prosemirror specifically warns against this: “If you're looking for a simple drop-in rich text editor component, ProseMirror is probably not what you need. (We do hope that such components will be built on top of it.) The library is optimized for demanding, highly-integrated use cases, at the cost of simplicity.”
Prosemirror’s schema, as documented, is more flexible than Quill’s. Prosemirror appears to allow anything, whereas Quill imposes some constraints. For example Quill requires all nodes to either be a leaf and cannot have children or a container and must at least one child. There cannot be a node that can optionally have children as is allowed in Prosemirror. In my experience the constraints Quill imposes lead to a more consistent and bug free experience across browsers. I will be curious to try out the edge cases I have encountered at this new 1.0 Prosemirror to see if it handles them the way an end user typist would expect. If Quill can benefit from a shift in the flexibility in its schema, it will do so.
Quill is far more battle tested. Slack, Salesforce, LinkedIn, Intuit and many others are using Quill in their main user-facing production products, not an internal employee only tool. Prosemirror has a great start with the NY Times but there is a large difference in adoption at the moment.