As far as the American and British English not being two seperate languages, I read somewhere in a linguistic article (or maybe one of my German teachers told me, I can't remember because it's been several years now) that in the next 100-150 years American English and "Proper" English (British) are going to be so different, with new slang terms and gradualy changing gramatical structures, that they will be considered two different languages. I know that that is a long way off, and I agree that they are only very distinctive dialects now, but still....

As for which you should learn, as an American from the south I can state quite fankly that most people from my backwards home town find it very difficult to understand a British accent. Keep in mind that these are people who say things like, "Ya'll gunna come on down to da farm fer sum chicken?" I've exagerated a little, but not much. I guess the point I'm trying to make is that if you learn an American accent and our dialect of slang then you will be better understood in America AND the English will understand you just fine.

I have a link for you, too. It's an online dictionary of British and American slang. I warn you that some of them are a little bawdy, but I think it will really help you out. http://www.peevish.co.uk/slang/