It really doesn't matter like everyone is saying. We usually do US first, just because we like it better then IOA. And usually we go to IOA in the afternoon, and it isnt as busy as US, or so it seems.
I usually do IOA first. Maybe I'm the odd woman out? I like to get the thrills done early in the day and enjoy a tamer experience in the heat of the afternoon or evening. Plus, there's nothing like seeing The Hulk looming in the sky and finally getting on it!!
on one of those days, i go to ioa at 8:15 and cross the fingers for the 8:30 opening.
i hit the hulk immediately.
the fog that is under the first dip early in the morning is a thrill for me.
later in the day you won't notice the fog effect.
i don't stay at one park the entire time.
i park hop back and forth on some days.
If you have FOTL you'll have plenty of time, no matter which park you do first. I don't think it really matters which one you do first, it all depends on how you look at it. Yes, IOA has a lot of rides that you can get REALLY WET on which is a good thing in the heat of the afternoon, BUT, if you're going in the summertime, afternoon thunderstorms are common and cause a lot of the coasters & water rides to be shut down for a while. Of course, because of FOTL you will have all the time in the world to walk to a different park !!!