I guess I'll swim against the current here. Personally I believe it depends on what you want to visit in each side of the park. For us, we are both thrill ride fans and HP fans. Therefore, we could have easily spent 2 full days at IOA. However, we did it in one (Christmas day when crowds are high) and we rode pretty much every thrill ride more than once and the other rides to our satisfaction.
We did not stay on site, but we arrived before the park opened and stayed right until they booted us out... Now it was a LONG day but well worth it.
As for US, well I'll be blunt and say we did not care for this side of the park nearly as much. The feel is 100% different there is no question, and depending on what you are looking for it can be quite fun as well, but any longer than 1 full day we would have gotten tired of it. However; once again we did it in one day, stayed off site and saw everything we wanted to.
Depending on the crowds on the particular day you are there staying on site and having use of the unlimited express passes can be a MASSIVE benefit. However, if the dates you chose are not particularly crowded then the passes are not needed IMHO.
In closing I'll say that my response is biased against staying on site, because our family does not care for hotels/resorts. We travel in our RV and love that lifestyle, it isn't for everyone.
Good luck and have fun no matter what you chose!