The very general response is: Swarm.js is CRDT and thus more async-friendly. It may survive failures all along the chain (db to server, server to client, client to cache, etc). It may synchronize with multiple sources: WebSocket to the server, WebStorage to the cache, WebRTC to a peer -- all at the same time.

That flexibility is paid by (1) implementing Lamport timestamps and by (2) limited use of version vectors (on handshake). ShareJS versions are linear, for example, but those linear versions are specific to a replica, as far as I can tell. In OT, version 3 here and version 3 there are possibly different.

CRDT/CT is generally easier to reason about than OT, esp. considering various non-standard situations and implications. That is mostly because CRDT (this flavor) employs "partially ordered log of immutable operations" while OT operations are mutable. That is the formal difference.