S3 uses a request signing mechanism. Each request is signed by the client and the signature is sent in the headers.

Rackspace Cloud Files uses a token based authorization. Authentication provides a token that is then sent in the header of each request. When the token expires, re-authenticate to get a new token.

The Cloud Files method is a little simpler for the user (IMO), but it also requires an SSL connection to protect the credentials.

Neither of these products allow non-public objects to be viewed in a browser (the browser doesn't send the correct headers).