The biggest difference I see is Identity management: https://atproto.com/guides/identity
That said...
> Usernames are DNS names. They are resolved using the resolveName() XRPC method and should be confirmed by a matching entry in the DID document.
> DIDs are an emerging W3C standard for providing stable & secure IDs. They are used as stable, canonical IDs of users.
W3C, you say? An "emerging standard" (quickly to be abandoned once deficiencies are identified), you say?