I'd say go to SeaWorld- you'll have an all around much more pleasant and enjoyable experience, which goes a long way toward having a memorable trip. I'm slightly biased because they're so much better to their pass holders than the other parks, but there are still many reasons for anyone to choose SeaWorld for a one-day excursion. First, I can guarantee the crowds at SeaWorld will not be an issue. Even if the park is packed the lines are never more than 30 minutes for anything, and there are so many nifty little things to see and do that the park never feels overrun (though watch out for the stampede that happens right after the last Shamu show of the day and any sort of nighttime lagoon show like Ignite); I've always encountered long lines and Times Square-like crowds at Universal, even during the 'slow' season. Second, the rides (in my opinion) are better and more varied, and overall there's much more to see and do at SeaWorld than at Universal, so regardless of your interests you'll still have a full day's worth of things to do. Universal has replaced nearly all of its attractions with screen-based simulators, and after one or two they all start to blend together and feel like the same ride. SeaWorld has a pretty comprehensive mix of different ride types, with kid-oriented rides, water rides (including a new one that should be open by then), roller coasters that each have some unique element to them, dark rides, simulators, a sky tower, and even paddle boats. They also have a bunch of shows, like 360-degree 3D shows, educational shows about animals, a couple of theatrical shows with and without animals, and if you go in August there will be several additional shows for Electric Ocean. Sadly, Universal has eliminated most of its shows, leaving just an animal show, a makeup show, and Shrek 4D if you want to count that; their nighttime show is currently out of commission. Third, there's the whole price thing: SeaWorld is just immensely cheaper, especially on the tickets.
Of course, that's what I think. What's more important is which park will make you and your family happy- I'm just trying to lay out some points for consideration.