I've done a bit in all the styles (not much in Arg.). In International, the partners can never loose body contact, and unlike the Americaln, the feet don't actually close on the last 'slow'. The way you bend your knees is a bit different than the two. American tango often has lots of openwork, like fans, etc., and even simple syllabus steps (like promenade) are a bit different. In both Am. and Internatl. dance position is the same. Argentinian seems to be a bit freer with the 8-count basic, but I don't know too terribly much about it.