My family went the week before Christmas last year and we had a great time. The first part of the week was not real crowded. The closer to Christmas it got the more crowded it became. So my vote is for the week before. I hope this helps.
We were there this past December from 12/18 - 12/23, IMO it wasn't overly crowded, although by 12/22-23 you did notice a big increase in crowd size. Overall though I didn't think it was too bad.
Week before...We spent 2 weeks there dec 2004 starting with 12/19 it definatley got packed by 12/24...I would say the week before was a smig less crowded...