Hi Ana!
You've picked two great times of the year to visit! They are definitely different, but both are great. It's really hard to advise between the two!
First, January 1st through 6th will have much cooler temperatures and will most likely have less swimming weather. By May, Florida weather has warmed up quite a bit and you'll spend quite a bit of time splashing in the pools and water parks! However, if your main focus will be touring the parks, the cooler weather might just be perfect.
As far as crowds go, I think you may have less crowds right after the New Year than mid to late May. With the early January dates, you'll most likely be able to see the holiday decor throughout the resorts and parks. This could be the perfect ending to the season for you and your family!
If your family has any Star Wars fans, they may be surprised to discover that Hollywood Studios may be transformed during select May weekends for
Star Wars Weekends. While the dates for 2012 have not yet been announced, last year's event did occur in May and June.
Hope this helps! Whichever weekend you choose, I hope its great!