You can't see both parks in 1 day, there is too much to see.
Universal has the Woody Wood Pecker play ground area, and it even has a kiddie coaster next to it.
This is a water park, with two houses with all kinds of hands on water contraptions to play with. Over each house is a huge bucket that fills with water. When the bucket fills, a bell rings and everyone runs to the spot where the water will fall. The kids get drenched like you wouldn't believe. Behind this area is a room with vacuum machines that suck up foam balls and shoot them all over the room.
Kids can easily spend 3 hours in this area alone, and you'll have to drag them away. Bring them here in a bathing suit, and have a change of cloths when they're done. The best idea is to have them wear a bathing suit under regular cloths. When you get to the water area, have them take off the outer cloths and shoes. There is a bench that goes around the outer edge of the play ground where the parents sit and put the cloths, noone will touch them. Bring a book to read, everyone just hangs out.
Next to that is a Barney sing alone show, and close by is an animal show. There is also a climbing play ground that has a water slide.
On the other side of the park is a Beatle Juice Musical show.
If you have two adults, one can watch the kids, while the other goes on a few movie rides.
US is the better choice and well worth the price of admission. My daughter is 9 and we spent 2 hours in that water area. I had to beg her to leave. Kids love getting wet.