With the number of Chinese people living in the U.S., Canada, and other countries around the globe, and with the rise of China as an economic and cultural powerhouse on the world stage, Chinese bilingualism today is more important than ever. Therefore, Chinese is becoming a popular subject of study in high schools and colleges around the world. But as you may already know, China has two major languages: Mandarin and Cantonese. So which one should you learn?
Well, first you need to ask yourself what your real goal is for learning a Chinese language.
Mandarin is the official language in China and Taiwan and is used by most of the Chinese schools, colleges and universities, as well as their TV programs, movies, and radio stations. Mandarin is also one of the six official languages in the United Nations. Even Hong Kong schools have begun switching from Cantonese to Mandarin education since around 1997 (when China regained its sovereignty from the U.K.).
Thus, if your goal is to be widely understood, you should learn Mandarin because Mandarin can be understood even in Hong Kong, Macau and Canton, and more and more Cantonese speakers are learning Mandarin nowadays. Cantonese is also somewhat more difficult to learn, as it has from 6 to 9 tones, while Mandarin only has 4 tones. In addition, because of its greater prevalence, it is easier to find Mandarin materials than Cantonese materials.
All of that being said, there is no need to restrict yourself to learning just one of these languages. Because all of the Chinese dialects use the same written characters and a grammar similar to that of Mandarin, it is not unreasonable to attempt to do both. But because of its greater ease, accessibility, and usage, I would recommend starting with Mandarin.