We usually do both parks in one day (myself, DH, DDs in early 20s, DS 17, DD's bf 24, and DSIL 28). The "kids" would say DEFINITELY do IOA. That park is an absolute must every time we go. They are "so-so" about US. We enjoy both parks. The water rides at IOA are terrific, and so is Spiderman. The kids love the coasters. We've been to Universal in LA and in Fla several times, so we don't mind not seeing all of the attractions. Last time we went, we started in US for Mummy and Shrek and then moved over to IOA, which worked out great. If you've never been to either, I agree that the 2-in-1 day would be best for you. If you must choose, refer to what "Lovethemagic" said. That's the best way to put it: US has more attractions like Disney. IOA has more thrill rides.