We did the last 2 days of our latest trip in early December at US. My older kids absolutely loved it, my younger ones kept asking "can we go back to Magic Kingdom?", so age does play a big part. Also, remember, to get the most out of the HP experience you have to get the park to park ticket (so you can ride the AMAZING Hogwarts Express train), which is significantly extra money, and for us, being able to ride that train was the only reason we got the park to park extra because theres really no way you can do both parks in one day. Also, I agree on there being WAY too many motion simulator rides in US, you have to take a dramamine to get through the day.
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But it very much depends on the type of motion issues you have. I get pretty bad motion sickness esp in cars and when we first went back to Uni a few years ago I was really concerned about the motion rides. So far knock on wood few of the motion rides at Uni have bothered me at all. I cant figure it out. I mention this because on our first occasion I medicated, felt like crap as a result and it turned out I didn't need it at all.