Which bitrates of Xvid and H264 give comparable quality?

[Nowhere]

Yes, I know, this depends also on source material and a lot of little factors/settings...but surelly it's not a heresy to say there must be some general rule.

So what do you think? When aiming for roughly the same quality, at the same resolution, how much lower in bitrate H264/x264 usually is, percentage wise?
And I guess it is possible that this percentage waries a bit with resolutions used/bitrate of xvid file?