We are using both American and British English most of the times and often overlook their origin and think the words as synonyms(eg. elevator/lift).Not only Vocabulary but also other aspects of language like grammar etc. differs.
When deciding IELTS(British English) /TOFEL(American English) lots of factors matter(which university you are applying etc.)along with that to get good scores you can decide which english(British/American) you would like to go for.