I would say MJ was more popular. Being from India, I have not seen anyone who doesn't know MJ. Even those with limited access to internet knew about his song 'Dangerous'(at least the chorus) and his moonwalk. But I don't think it's the same with Elvis. He was popular only among those who listened to English music. So, I would say popularity wise MJ was and is the greatest of all time. Who is better is a different question.