Video is almost always offered in both formats. If you are using either Netscape Navigator or Microsoft Internet Explorer on a more recent Windows or Macintosh system, your browser should be automatically set up to play video files in your platform's native format.
Quicktime files tend to be slightly better quality and somewhat smaller in file size.
The Mac version of Netscape Navigator and Microsoft Internet Explorer will automatically use MoviePlayer to play Quicktime video. Quicktime files tend to be slightly better quality and somewhat smaller in file size.