One is no more pure than the other. There are multiple accents in English, including several in the USA, several in Canada, the UK, and so on. The most notable difference between American English and British/Australian/Canadian English is how we spell words. In America, we spell 'color' instead of 'colour', 'license' instead of 'licence', 'center' instead of 'centre'.
Some words also have different meanings in American English than they do in British English, but that's true of many languages that have spread throughout the world. Language evolution is more than just change over time; it's also change over distance.