ProseMirror (used by tiptap) is way more stable and comes with different types of collaboration support (OT & CRDT). Data is stored as JSON as well. But you can import/export good old HTML too.
Some things are even easier in tiptap, some are better supported (collaborative editing), and Slate.js is only available for React, but tiptap is framework-agnostic.
If you’re happy with Slate: That’s fine, it’s an amazing editor. For some use cases, tiptap is probably better suited, though, but that highly depends on your use case.