The main difference is that young American English slang is usually just co-opted Black English slang from 15 years ago. British English has a wonderful history of slang that American English has never even begun to have. For example, the height of American English slang paradigms seems to be to invert the meaning of a word (``bad'' for ``good'', etc.). The height of British English slang paradigms seems to be to rhyme the word you're slanging with a multiword phrase and then drop the last word -- the one that made it rhyme in the first place (``butcher's'' for ``butcher's hook'' for ``look'', etc.). Judge for yourself which is more interesting.