Mandarin is the national language of China and Taiwan, and is the dialect you should choose if traveling to those countries, or if you're learning Chinese for academic/business purposes. China's large population makes Mandarin the most spoken language on the planet. Cantonese is the primary language of Hong Kong and China's Guangdong province. It is also the most common dialect spoken by Chinese overseas. Choose Cantonese if you're traveling to Hong Kong, doing business in Guangdong (a major economic region), or if you wish to speak with the majority of Chinese in the US, UK, Australia, or Canada. Difficulty: Mandarin is more structured and a bit easier to learn, while Cantonese has more tones and an ever-changing variety of slang terms. However, this characteristic also makes Cantonese a rich and enjoyable language to study. While there are similarities between Mandarin and Cantonese (learning one can help with learning the other), attempting to learn both simultaneously would be very confusing and inefficient.

Thank you for the answer, 崔雅倩! In a video that I watched about the Chinese languages, the pronunciations of the languages in the example were not actually so different than each other. Therefore, I thought that it might be easy to learn them by simply alternating the vocabulary in some extent. But, I think it is different tones and pronunciations that makes it a bit difficult to learn another. I think, the grammars should be similar, though.

Even if you can speak Mandarin very well,it's still a little hard to learn to speak Cantonese ,Teochew or Taiwanese,but you can understan the written form of other dialect. In fact ,I think that learning Mandarin well is enough,because no matter which dialect speaker can understan Mandarin.