I think it depends on your plans. If you are more likely to be going to the US or communicating with Americans or people who have learned American English, learning that kind of English seems like a good idea. If you think you are more likely to visit the UK or communicate with British people, learning British (English) English seems wise. I know a man from Czechoslovakia who'd learned British (English) English and had a lot of trouble being understood in the US! But his accent would have been an advantage in the UK. Two Germans I knew had learned American English and were quite easy for me (a Californian) to understand. But someone from the UK might have had trouble understanding what they were saying.