If your girls are thrill-seekers, then I would definitely recommend USF/IOA over MGM. There's no comparison when you consider the thrills at IOA to those that can be had in MGM. IOA wins hands down with the Dueling Dragons coasters, the Hulk, Spiderman and Jurassic Park. The shows (Poseidon's Fury, The Eighth Voyage of Sinbad) are top of the line.
In USF, there's the new Revenge of the Mummy, Shrek 4-D, Men in Black Alien Attack and Back to the Future. For shows, you've got Terminator 3-D, Beetlejuice's Graveyard Review and Animal Planet LIVE!
If you buy a 2-day pass, you can hop between the two parks easily (they're a short walk apart). The 2nd day never expires, so you could save it for another trip. Ticketmania sells them for $106.
For the cost, I think that I would choose Universal instead.