Depends:
They both share code. Divx 5.05 and Xvid are pretty much head to head at the moment, biggest problem is, is that Xvid is actually illegal in binary form in several countries as it violates software patents (which are stupid). Source code is distributable though.
Also, they are not really intercompatible. You can compress an xvid so it runs on earlier versions of Divx, but the quality isnt as good as a full xvid encode. Xvid can run Divx 5.05 movies, but the decoder isnt built for that, so again there is quality loss.
I personally use ffdshow for every MPEG4 based codec except for divx 3.11a, which I use the original binaries for. I encode in xvid OGG.
As for GP32 encoding, I find that Divx 4 gives better quality for some reason. I just cant seem to get the settings right for xvid.