Dear XXX:
Knowing that you are troubled by the choice between British English and American English makes me feel so sorry.So I am writing this letter to you,hoping to give you some clue.
Actually,the difference between British and American English is not very big.They mainly differ in pronunciation.The spelling,vocabulary and grammar between them have very small difference.For instance,Britains prefer to say "Have you got",while Americans say "Do you have".
As for how to choose between them,I think as a language,all you have to do is to use it to communicate fluently with others.So as long as you are capable of using it,no matter which you speak,it's all right.But if you want to be noble,choose British English,while if you want to be casual,American English may be suitable for you.
I hope what I mention will do you a little favor.Best wishes to you.
Yours,
XXX
希望回答能够对您有所帮助,祝您天天开心!