I've spent a good amount of time in the Florida parks, and plan to go again this year
If I had to just pick one, then I would say Universal. Only because Islands of Adventure was pure fun, easily one of the best theme parks I've been to. The rides were awesome. Universal Studios was OK, but on the whole it just felt dated. I think we'll be sticking to Islands of Adventure only this time.
Disney has it beat on variation and quality though. Fewer thrills, but with each park they focus on the experience. Kids will LOVE the Magic Kingdom, Disney have created the perfect park for their fans. Epcot and Animal Kingdom are both great for quieter days, and if you have a ticket allowing you entry into whichever park you want then they are a must. The World Showcase at Epcot in particular is interesting, but there is less for kids to do. MGM is definitely worth a visit if only for the Aerosmith ride and the end of day performance which blows the other parks fireworks away.
If your down there though, don't forget to give Busch Gardens in Tampa Bay a look. It's a bit of a drive away, but damn their rides are impressive.