When ESL students try to take on an American accent it usually sounds absolutely ridiculous. Stick with the British as it sounds more natural. And when I say British I don't mean a regional accent , I mean RP/formal pronunciation based where each and every pronounced consonant is heard clearly and each syllable is enunciated correctly. But at the end of the day, it depends on how serious you are about the language.

Given that you are probably going to need English to communicate with other ESL speakers, neither. Just English. Idioms, slang, vocabulary specific to one English-speaking country - these are interesting if you are studying for love of the language and useful if you want to live in an English-speaking country, but they will be totally unnecessary in most situations.

Daft question.