i would personally go to US first because Revenge of the Mummy is located there and that's my absolute FAVORITE ride; but there is also a TON of great rides in IOA like:
1. spiderman
2. dueling dragons
3. hulk
4. river adventure
5. ripsaw falls
plus walking around marvel superhero island, lost continent, and suess landing!!
nope, just can't decide.... we always let Bubba decide.... i could ask him for you if you want....
what time do you arrive? is there anyplace you want to eat at? ie: we arrived just before lunch.... had a plan to eat at Studios and IOA depending where we were.... also, if you arrive early, it's not as crowded or hot, so do IOA first (outside stuff) and then go to studios where most stuff is indoors....maybe that will work?
i started going to universal in 92 and only the studio side was opened then.
i like both parks but always start with the studios for the beginning of the day. i start there, do IOA before noon and then hit the studios again before i return to my room at hrh.
I think IOA is by far the greatest park in the world out of any park anywhere, so I always do IOA first. But I never skip the main Universal park either, it has some great things as well, the Mummy, MIB, list goes on and on. You should probably make equal time for both, but I would do IOA first if you want to be really taken away to another time/place/dimension whatever, it really makes you feel like you are in those worlds.
based on this info alone (btw, parks will open at 8:30), I would say either park first is fine.
studios is more nostalgic if you'd rather start there. you can get good seats on hulk and dd if you go there first.... i will ask bubba after he's done in the shower and let you know his answer