I have heard that you should avoid the parks the week betwen Christmas and New years due to crowds. We ahve ben to both DL and WDW for X-mas many times (usually leaving weither the 24th or 25th for home) and the earlier the better as the closer that you get to Christmas the busier it gets. So if you are set on visiting one of those periods I would go before Christmas.
No comparison. It will be MUCH less crowded Dec. 21-23. We went on December 21 and stayed through Dec. 26 when my DS was doing the College Program. Each day was a little more crowded than the day before.
I was there from December 20 to 26 in 2009 and it seemed like it got more crowded each day that we were there. I suggest going before, rather than after, Christmas.