"Which compresses smaller?" is not a valid question as you can determine the bitrate freely, just like with DivX, so you can make your 18 hour movie to fit into 10MB with all of the formats. The real question is: "Which format looks best when I have DVD source material and I want the movie to fit into two CDs?" or something like that. And answer to that depends pretty much on the size you're aiming to -- not that if it would matter if you're planning to make VideoCD or SVCD, since VCD doesn't support other than standard bitrates. IMHO, regular movie in SVCD/MPEG-2 format encoded so it takes exactly three CDs, is the ideal solution.
