Chinese: Mandarin or Cantonese?

Mandarin is the government sanctioned language of China. It is spoken in all major cities. Out "in the boonies" some people won't know Mandarin but even there the public officials will speak Mandarin.

Cantonese is the local dialect of Hong Kong. It is not spoken in Beijing or anywhere outside of Hong Kong yet still powerful for economic/business reasons. Sounds harsh, unclear and "uncouth" to Mandarin speakers.

If you speak Cantonese you can understand Mandarin.
If you speak Mandarin you cannot understand Cantonese.