My vote is to take them to US.
We just got back from 3 days at US/IOA with 4 kids (10,9,7, 9mths) - not all mine. Our 9 mth old could only ride the Caro-Seuss-el and One Fish, Two Fish (although he fell asleep just before we were to ride so he didn't ride One Fish) at IOA. If the child can't walk 3 steps w/o help they can't ride The Cat In The Hat or ET (at US). Other than 2 shows and the 2 rides mentioned, 9mth old couldn't do anything else which means you are child swapping (takes up a lot more time) or someone isn't riding.
He (9mth old) could do most things at US. I think he sat out of BTTF, MIB, ROTM, ET and Woody RC. Not bad considering all of the other attractions.
Have fun. We will be back in May!