Going to China? The safetest suggestion is to take Mandarin, if you can pick one language at a time. :-)
These are two SPOKEN styles/dialects of Chinese language. As an official spoken “dialect”, Mandarin is widely used in Mainland China, Taiwan area and Singapore. Cantonese is specifically for Hong Kong and some overseas Chinese communities.
One of the most important questions that many new potential Chinese learners may ask themselves is: Should I learn Mandarin or Cantonese?
If your goal is to be widely understood, you should learn Mandarin because Mandarin can be understood even in Hong Kong, Macau and Canton (the main regions who still speak Cantonese), and more and more Cantonese speakers are learning Mandarin nowadays. If you really want to be able to connect with people from Hong Kong, Macau, and Canton, you can still consider learning Cantonese. But you should still know that Cantonese is often seen as more difficult. Its use of “tones” can be even more challenging to western speakers than Mandarin.
It’s easier to find people to teach you Mandarin, including some of your Chinese friends who have learned “proper Mandarin”, and who may be able to teach you the language step-by-step; More Mandarin learning materials are available from all sorts of sources and at a variety of levels; With the current economic growth of China, it’s likely that Mandarin will be a key language of the future.
A link for you if you are interested in learning about the difference between these "two languages":http://www.actranslation.com/chinese/chinese-manda...