One of the bigger differences is that the Open divisions will have a similar (if not the same) scoresheet as the senior divisions, where tumbling is weighted heavier. So some teams with slightly weaker tumbling may decide to go International Open as those have traditionally not had tumbling weighted very much on the scoresheet. I'd say a big reason we're seeing so many US teams choose Open is because they will have top 10 move on to finals at Worlds regardless of country opposed to top 3 from each country. It was kinda expected that would be the case when the divisions were announced.