Mandarin is the "standard" language in China. It is the most commonly spoken throughout whereas Cantonese is just the most widely spoken dialect.
You see, in China, there are many different dialects depending on where you are from, most are just variations of Mandarin (such as the northern dialects and An Hui dialect for e.g.), however, there are some that are very different to the standard Mandarin, such as Shanghai dialect or the most commonly known, Cantonese. This is because around 25% of Chinese people (mostly in Southern China) speak the dialect, and a large percentage of them cannot speak the standard form of Mandarin. Therefore, it has become known as a Chinese "language", though technically, it's not.
Furthermore, the Chinese characters were simplified some time ago to make things MUCH easier but for some reason, people speaking Cantonese and Taiwanese still use the traditional forms. I would like to point out here that even though they are sound quite different, all Chinese characters have the same meaning wherever you go! So reading is never a problem, even if listening is.
Learn Mandarin. You will reach more people that way. UNLESS you are planning to move to Guang Zhou or Hong Kong.