I was just researching the same thing. We're heading there during spring break season, and there is basically NO good day to go!
Anyway, some websites indicate that Sat, Sun, Mon are the best days to go to Universal. I think the idea is that those days tend to be the beginning of people's Orlando vacations, and most seem to start off at Disney.
HOWEVER, there are some websites out there that forecast specific calendar dates for you, taking into account other special events that may impact crowds. Universal does not release their hours as far in advance as Disney does, so you may have to wait closer to your trip dates to know for sure. Google Undercover Tourist and see what you can find.
Second Star to the Right, and Straight on til Morning!