We were there the first week in Dec 04, and just returned from another trip last week/end of Jan and first of Feb. Between those two trips I felt the crowds were slightly lighter Jan/Feb. Not a big difference in crowd level but noticable nonetheless.
However, Jan/Feb had several ride closures, not to mention the castle work being done. Early Dec was decorated gorgeously for the holidays with only IASW being closed (that I recall). I'm guessing repair work gets done during the early months of the year, after the holidays are over, b/c it's a low crowd time of year. Because of this I would prefer to go in early Dec over Jan/Feb, even with the crowds being slightly larger.
Hope I didn't confuse you!