If you only have one day, I'd try and do both parks. I'd start at Islands of Adventure, just in case there's some off chance Harry Potter is open, otherwise I'd get all the Marvel attractions done then work clockwise around the park, doing whatever interests you.
IMO, the only attractions really worth the time if you're trying to see both parks in one day are Spiderman, the Hulk, Ripshaw Falls, Bilge Rat Barges, Jurassic Park, Dueling Dragons, and the Cat in the Hat.
Then at Universal start on either side, but definatly try and see the Terminator, the Horror Makeup Show, the Simpsons, Jaws, Men in Black, the Mummy, Rip Ride Rockit, Shrek, and if you have time, maybe ET. (And maybe Jimmy Neutron also.)