Fortunately, British and American Englishes are not too far from one another in terms of vocabulary or pronunciation. There is no risk of serious misunderstandings. It might be better to pick American if your goal is to become proficient at English. That's because some parts of colloquial British vocabulary tend to be not well-known to most people who speak English as a second language or even North Americans whereas the British usually have enough exposure to American English to know the American counterparts of those words.