Both if possible. Now that you have moved beyond intermediate level, you should make sure you are familiar with both types.  This step normally happens naturally when you have a change of trainer.  Most training teams, including ours are composed of teachers from every continent.

Listen to lots of accents

Generally speaking there are two types of English when it comes to standards of vocabulary, spelling and pronunciation. But remember there are 25 countries with English as an official language, and therefore quite a lot of accents right across the English-speaking world. Even within Britain and Ireland where there is one written standard, there are a multitude of accents. I come from Ireland where the English spoken is really neither British nor Irish. It has something in common with both. It is really good training to get practice with a variety of accents during your listening training. The easy availability of media actually makes that task quite easy.  It’s also interesting and good fun.

Slang doesn’t travel well

The world is getting smaller and despite the small differences in accent and dialect between the English-speaking nations, generally nobody has any problem understanding anyone else. It’s mostly a phonetic difference  between each nation.  It is important to be conscious though of using colloquialisms.  For example, if you are in America and renting a car in New York.  If you ask for one with a big “boot”, the rental rep is probably going to know what you mean even though that is a British English word.  But if you start to use London Cockney slang that you have learned in your English training, you are unlikely to be understood. 

Keep to one standard when writing

In your correspondence and marketing communication for your products, pick one standard and keep to it. Select a dictionary so that the spell checker will keep everything consistent when you are typing. If you’ve mostly learned to speak according to the British standard, it isn’t a mistake if you use the odd American word in front of an audience from Britain, Ireland, Australia or New Zealand.  That is how language is evolving anyway.

Neither standard is “better”

Neither British nor American is superior.  They are just different. Lots of learners tend to identify best with one. That is usually due to personal experiences of travel and making friends in particular countries.  That’s really great. If you take on your teacher’s accent, that is a huge compliment. If you love New Zealand or Wales or Ireland and therefore take on the accent, that is absolutely great.  Accent isn’t as important as being able to communicate confidently and clearly with your internationally business colleagues.