I don't think one is better than the other. Bear in mind that there are many variations within England and within the US, and that these countries are only two of the many English-speaking nations of the world. But the choice that most learners face is that between British Received Pronunciation (also called by some BBC English, or the Queen's English) and General American (also called by some Network English or Standard American). Both of these reference accents are highly intelligible and well-known around the world, and they have more similarities than differences.
So it's only a matter of personal choice. If your teachers teach the American model, it's wise to follow that one. If you listen to the BBC, or have an English friend with whom you can talk regularly, choose RP as your model.
- For Teachers