I have a daughter just a touch younger and not a Harry Potter fan yet. She loves all of the princesses and even I know every "new" Disney movie by heart. 10 out of 10 times she picks universal over Disney, don't short change it, try to get the second day! I get the Disney pixie dust that all DIS fans talk about, but there is plenty to go around at Universal as well.
A little tid bit that got me hooked on UO that I use often. One night stay on site gets you two days worth of Express Pass. I was staying off site and didn't expect to actually sleep at Hard Rock, just booked it because it was "cheaper" than two days of Express passes. So we took an overnight bag so we wouldn't look suspicious. As soon as we hit the lobby and walked the staircase into the pool area, my kids were in complete awe. We have done our share of 5 day Hard Rock trips, but even when we do a Sea World or DIS vacation, we always but two day tix with a one night stay at HRH. With the "worth more than gold" Express passes and two days, we always get to see what's new and hit our faves as much as we want.