What does 'popular' mean? Number of people that speak it? Number of countries that speak it?
The USA has a large population, so probably wins on total speakers. The size of the British Empire probably left a legacy of many countries that speak with more of a British accent.
In terms of what people that are learning want to learn - it probably depends who they want to talk to most, and where they want to visit. If you wanted to get a job in the USA, you might want to cultivate a US accent to get accepted more easily.
But more importantly - it's just an accent. And as a non-native speaker, your own accent will be much stronger than any accent you try to cultivate. To me - your accent will sound more Arabic than it would sound British or American, unless you spend a lot of time in that country. And both accents are mutually understandable. You will be understood whichever accent you try to emulate.