Debian Sid is the original rolling release. Its advantage is is that you are on a rolling-release distro but it's still Debian, so all the system knowledge you accrue will apply seamlessly to production Debian servers. Most rolling-release distros, on the other hand, are typically not used on production servers.
Sid is not a rolling release. This is a common misconception.
In preparation for a new stable release, it goes through what is effectively a package freeze. Maintainers are discouraged from updating packages in Sid in order to focus on the new release. So almost no new updates are uploaded to the repositories until Stable is finally out.
That's not exactly a hard rule, just stuff maintainers do because it is far easier to still have normal flow of unstable -> testing for the freeze period, and keeping 2 ("actual unstable" + "unstable on the way to the new release") would be a big burden for maintainers.
Also, packages are tested in sid/unstable and migrates to testing which then becomes the new stable when it's ready.
And packages are tested across releases, enabling one to dist-upgrade to a new version of the system easily. I think it's piuparts that does this job of testing packages among themselves too.
