Certainly divX is much better than 3ivx as far as encoding goes. Much better playback fluency for a start let alone quality & size. For personal use, I found ZyGo to be very good. As good as DivX if not better. But it might depend upon particular file! ZyGo is freeware I think, as a 3rd party QuickTime component you can downld via QT. But my tests have been very positive generally as for size & quality & playback fluency. But I don't know. Overall DivX is more than likely prolly better.

I also haven't tried DivX 6, but it is only bound to be better than previous versions (saving for some bugs in some version releases.)