Universal/IOA are great in December. We try to do it at least once every year. The holiday shows, Grinchmas at IOA, the Macy's parade at Universal. Special holiday Luau at RPR (close to Christmas).
I definately say go...you will have a blast. Adding Universal after your disney trip is a good idea...since staying on site at Universal, you will have front of the line access and you will be so spoiled.