As an American, I can tell you that it is really hard to characterize the American school system. It depends a great deal on where you are going. If you are going to the US, then I would say you get a much better education in either a wealthier area or in a private school. That said, every city has good schools and bad, and the big cities have the best schools in the country- NY, DC, LA, Denver, etc. State schools outdside Washington were in the top 100 schools in the country this year. An international school in the US would be expensive and would house mostly non-English speaking kids, but would be diverse.
Outside the US, American international schools vary widely and are dependent largely on the country they are in. We, for example, live in Germany, and the American International schools here are roughly one year behind their American counterparts, because Germany does not require entrance until 6 and the cut off is June 1, so lots of kids who would start kindergarten with fall birthdays are a year back. Also, the German system is not rigourous in the first couple of years, and more than half the kids are coming into school without English. This makes it hard for kids who are already reading and speaking English in the early years programs, as the schools focus on the kids with language needs. At the secondary level the IB program is much more uniform country to country. At the secondary level, International schools have very good extracurricular activities, travel, and emphasis on community awareness. The IB is quite a rigourous diploma, certainly moreso than a regular American diploma and it targets more applied thought than the standard AP tests do. Keep in mind that while the American system starts a bit later, the R-2 years of the British system would roughly be covered in 2 years, rather than 3 in the American and from year 3 (American 2) the curricula are very similar until year 7, when location and individual school resources make American schools very diverse. This numbering difference may mean that the kids drop back a year to be with their peers or at the appropriate level.
Outside the US, the international schools do provide moms with a lot of help and networking which can make settling in easier, and the kids are all moving about so they are used to welcoming new kids and making friends very quickly. That said, turnover can be as much as 50%-60% in a given year.
We have our son in a British school, as he is in primary, and we found that for the early years the British schools are more uniform, have a dependable curriculum (which is not always the case overseas) and they put a bit more emphasis on memorization, which I think is still important. The early years in the American system are hands on and abstract to a fault, often emphasizing creativity and expression but not giving the kids enought material (memorized dates/facts/ numbers, etc.) to be creative with. At the upper level, however, the American system has a lot more room for diversity of kids, and one is not locked out of higher education if one slacks at 14. The system is much, much more forgiving to teenage inconsistency.