I found the same problems to be present under multiple OSes and versions of both browsers, so it is certainly a site issue. Since both the site and Quicktime are developed by Apple, you might want to ask them why they have apparently failed to create legible content on some pages, while succeeding on others, using their own technologies.
The differences between browsers likely boil down to Quicktime plugin issues, but the fact that some Quicktime content works and other content does not indicates that the Apple site developers created flawed content.
I've found out that some Quicktime trailers are choppy in IE, but not in Firefox and Opera.
Playback is jerky in IE, but not in Firefox http://trailers.apple.com/trailers/disney/aliceinwonderland/ (Also moving the mouse while video is playing in IE causes it to be much choppier and CPU usage to shoot up for some reason. But it's jerky even when I don't touch the mouse.)
Smooth in both IE and Firefox, but all links on the page are mysteriously "blued out" in IE even though they still function. It's just the text is blued out. http://trailers.apple.com/trailers/summit/newmoon/trailer_md.html
Plays fine in both IE and Firefox. http://trailers.apple.com/trailers/wb/sherlockholmes/
Any rhyme or reason to ALL this? I wonder why the text in that one page is blued out in IE.