DivX Pro is only good for making DivX files (ending with .divx or .avi) Quicktime pro can however read a lot more file types and evport to more and i feel that there are more options in the export settings in QT-pro.
QT-Pro is basicly a VERY low vevel edit suite, cos you can chop and change movies, add and remove parts, then export to a lot of different file formats
as far as i know DivX is basicly an mpeg4 converter, but because its a tweeked mpeg4 file with a wierd (wierd to quicktime anyway) encapsulation only computers with Divx can watch the files.
hope this helps.