Replay Music does not record via stream, it requires the Sirius Online tuner and records the decoded analog, then compresses to mp3 (or many other formats). But it will automatically create a new file when the song changes (silence level/length needed is adjustable). After splitting the file, it attempts to identify the song, then tag it with Title/Artist/Album. Works with Rhapsody, internet radio, etc., too.
Replay AV will login to your Sirius account and record directly via stream with no quality loss when converted to mp3, but it does not have the splitting/tagging functionality of Replay Music. It is convenient for recording shows (like Howard Stern), because you can schedule the login/logout time and it will wake up your computer and record unattended.