My kids have loved IOA from a very young age. Out of the 7 parks (not including
LEGOLAND since we have yet to go), IOA is their second favorite and has been since my oldest went for the first time at 3 years old. The last time we did the US part, the kids were 11 months, 3, and 6. We ended up doing just half a day at US then and 1.5 days for IOA. That was before the Harry Potter area opened and they love HP.
For us, Disney isn't better or worse than Univsersal, it's just different. My kids could spend a week happily in Epcot, their favorite park, but after three trips where they hated Animal Kingdom, I finally learned my lesson! US had a few things they loved - like Men in Black was one of my oldest's favorite rides even at 3.5 years old. And my middle child still talks about "Llama, Llama, Llama" of Jimmy Neutron, which is now Dispicable Me. But all in all, IOA was a MUCH bigger hit. They could have easily spent an extra day in IOA but we really needed a break day.
Keep that in mind with young kids. On that trip, we happened to do 7 parks in 8 days. We're rope droppers and then keep going until one child acts like it's time to leave, which is around the dinner hour. We took a day off in the middle for DTD and the pool - we wanted to make sure we had planned pool time! Taking that day off also meant we could do a later night the night before. Add in the last night, and we had two nights that we would keep them up a little later. (Like 6:30 Fantasmic or 8pm Wishes, not 9pm Illuminations or anything super late). So, if you're planning a park every day, then I'd instead suggest taking an off day in the middle even if it means 2 days at IOA/US instead of 3.
Don't worry about where it is in your schedule. We usually do it before or after since we do a split stay with it. Once, we did just one IOA day in the middle of the trip and it worked out well too. Even the Disney parks are all different from each other and we like to mix those up. We use crowd calendars to help us with the planning.
