If you plan to post to youtube (or other flash based system), you should find out their maximum file upload size. Then export your QT movie to that file size. Since your file is not getting streamed, it doesn't matter what setting you choose - remember, the service is going to re-covert it to their standard video format anyway.
So what you want to do is upload the best quality video as possible. They recode it, and post it as a streaming whatever-file (flash, mpeg, etc).
Uh, to answer your Q... it doesn't matter which you choose. As long as the quality is as best as you can get it while adhering to the service's restrictions - usually a file size limit.