Do exactly what Princess said. Stay in a universal hotel, get a double park bonus ticket. And Fastpasses (so, so worth it!) You'll have an early start - straight to Harry Potter. Escape from Gringotts is the most awesome ride, get the Hogwarts express to Islands of Adventure and do the HP ride there. They are amazing. Loved both Universal parks. My youngest is 8 and hates all rides, my others (oldest 13) couldn't get enough.
My tip is of you don't get in early, do the park in reverse, so head right and do the Terminator etc, then work up to Simpsons and to HP that way.
We left there yesterday and spent today at Disneys Magic Kingdom. It is exactly like Disneyland Paris and we were very underwhelmed. After Universal it was a real anti climax. Had breakfast at the beasts castle and that was really good, but everything else was a bit 'meh'. Maybe we peaked too soon 