The Quicktime format is just a media container, like .avi files. As such, they can use any number of different video compressors, some better than others.
The trailers you've seen have been compressed with the Sorenson codec for Quicktime - tests I've seen of it (for example on the
www.3ivx.com website) have it lagging far behind either DivX version, in terms of quality for a given bitrate.
The MOV format itself is better than AVI from a technical standpoint, as far as a media container goes. I'm working to port the XviD (
www.xvid.org) codec to work with Quicktime, both on macs and pcs, so hopefully the format will be used a bit more. One thing really holding MOV back is a lack of tools as good as, say, VirtualDub.
-h