I did both last year- Christmas eve was definitely better overall, though Christmas day was light early but got packed in the afternoon; however, I think all bets are off this year since DL will be open til midnight Christmas eve. (last yr it was only open til 9pm)
I agree that Christmas Eve felt less crowded. It was crowded, but manageable. Managed to finish most of Disney California Adventure well before 6 PM (had been in the parks since 10 AM) so it wasn't too bad.
Christmas Day wasn't as crowded as I expected. I recall that a section of Disneyland Park had a power outage late in the day which made unaffected areas of the park more crowded. I managed to go to Tom Sawyer Island and Davy Crockett's Explorer Canoes though which are attractions I usually cannot fit into my day schedule.
On both days that I was in Disneyland Park, I remember being surprised at how the park wasn't as crowded as I expected.
I too have been there on both, and I would agree, Christmas Eve is the least busy, but don't be fooled........less crowded compared to Christmas Day is still pretty crowded.
Least to most crowded:
Christmas Day before noon
Christmas Eve
Christmas Day after noon
New Year's Eve (I know you didn't ask. Just threw it in as a bonus.
I too have been there on both, and I would agree, Christmas Eve is the least busy, but don't be fooled........less crowded compared to Christmas Day is still pretty crowded.