I have danced both internationl and American, now just international.. (smooth/standard)
A few of the differenes that I see, is that in American, you do much more side by side work, and will break your dance frame, while in international, you always maintain dance frame and contact. ( I am sure I will be corrected on this)
Also in American there appears to be much more flexability as to the dance steps you can do at each level... In International they are much more defined , that is prior to the novice level.
Also, I have found that dancing internationl, has resulted in our American technique to improve greatly... The two techniques are quite transferable.
Another interesting point... We live in New England, and at an amatuar level there is very little competition in American, while a great deal in Internationl. Recently, we did a competition in Louisana, and the deck captian mentioned to me that it was a treat to watch international dance, as most people in this area only dance American.....We found his comment interesting.