Beta vs RC: What's new in the Windows 7 Release Ca...
Microsoft has paid close attention to Windows Media Center during development, and the work shows. The version accompanying the Release Candidate has been tightened and polished. It provides more comprehensive driver support for TV tuners and other devices. Integrated support for playing Quicktime MOV files in Windows Media Player (and hence Windows Media Center) also gives you another reason to switch.
Beta vs RC: What's new in the Windows 7 Release Candidate?