We went last summer with a 4 and 6 year old (boys) and spent substantial amount of time at both parks. It was very hot so that definately effected what we did. This is what stands out in my mind from a year ago....
IOA- kids liked spiderman, pirate ship climbing thing, suess land, jurassic park area playgound and flying dinosaur ride, small roller coaster.
US - jimmy neutron ride (I think 2 year olds can ride in the front where it doesn't move), fire hydrient that opens near the irish pub - we were there for over an hour!, shrek, barney, curious george water sprinkers, another play area where eveything was larger than life, jaws (although they both hated that one!), small roller coasters that they enjoyed as well, and ET (rode it about 10 times!)
If mom and dad like thrill rides, I would go to IOA only because then there are rides for you as well (and some really good ones!!). If your intention is to only keep the kids entertained than I believe there is more at US.
The tickets are free so ENJOY!