Do you think it matters what kind of English you learn?Why?

it does matter, i think. by the kind of English that you speak (language variety, as we define it), people know who you are.
if you can speak english in a totally british accent, you look better educated or more serious or eligant...but just look so. if you speak english full of american slangs or in an american accent, you look more natural and casual.
but this is just the way it seems to be. it doesn't matter what accent you adopt. what matters is the person that you present to others by the language you use. in other words, the language is part of your identity, even though it is a foreign language. and that's why i don't encourage my students to learn slangs.
there are other kinds of english too, like formal and informal, that are specialized in certain careers, locations, social status...
on the whole, by talking about what kind of english should be learnt, we need to consider some more factors besides accents, such as careers, locations and s