If you aren't living/are planning to live in either places, however, my advice is to get used to the SAE accent first. From my experience, SAE is a clearer accent and so is much easier to understand and learn, and so is a great segway to learning how words are pronounced in general. Then, when you feel more comfortable with SAE, you can try to delve deeper into other accents, such as the standard British accent, regional American accents, etc.
I hope it helped, and I apologize if there were any grammatical mistakes. :)
No matter the «accent», just learn proper English and it will be understood everywhere. British English has, according to me, a more complete overview of the lexical, grammatical and phonetical parts to the English language.
But the «accdent» should not be the first and most important side to learning a new language.