Neither is better, because language is just a bunch of sounds put together that some people thought would be good for communication purposes.
The average American cannot understand foreigners at all if they do not attempt to speak with a standard American accent like one you would hear in the Midwest, or even large parts of Canada. I struggle very hard understanding Asian ESL speakers who use the British accent for English, so if you're going to be working in the states or with mostly Americans, go with American teachers.
There are some accents of English emerging that are almost becoming their own language. Those are the types of English teachers I would stay away from, that are going to teach you such a highly unique style of English that only one or two regions would understand you (for example, Welsh valley accent, Geordie accent, certain Irish & Scottish accents). When I was learning Korean, I wanted to learn the Seoul dialect that millions of people would understand, not the Pusan dialect or the Gwangju dialect - just as an example