Well, many people seem to think that XviD is much better. Many people agree that XviD has much better quality and loading speed than DivX. But the only thing I know is that XviD is open source, and DivX isn't. But my computer works perfectly fine with DivX, so I have no argument here.
However, someone on Yahoo answers already asked this question, and it has been resolved. I think that question might be able to give you more info than I am. Here's the link: http://answers.yahoo.com/question/index?qid=20080622062018AA3zoIi