/var/log/Xorg.0.log and /var/log/Xorg.0.log.old have timestamps on (most of) their lines, so you are not able to compare them directly.
This is the perl one-liner, that I just used for replacing the timestamp by white space:
perl -pe ‘s/^[……….]/[ ]/’ /var/log/Xorg.0.log > /var/log/Xorg.0.log.NODATE
If you apply this to Xorg.0.log and also to Xorg.0.log.old, you are able to compare the respective results.