x264 Is definitely better than DivX/XviD.
Here you go, x264 vs. XviD on a quite tough source.
Unlike with x264, I couldn't tweak the settings to all hell to make them look good for this one source with XviD. But the speeds are 15 fps for x264, and 23 for XviD. (So it's not like I used "fast" settings for it.)