I really doubt about Harregarre's natioanality because his answer is so good:)
Yes, Chinese Mandarin (In China we call it Putonghua, means standard language, which compares to those dialects.)
I suggest you should learn Mandarin first, just as they said.
With Mandarin you can visit almost all round China, also available for the Southern China like Guangdong.
Cantonese has its advantages, especially in business affairs. If you want to run your business in Guangdong, Hongkong. You will get suprising benefits from Cantonese. Believe me. People living there are famous for their wealthy, clever, creativity, and great enterprise.
According to the data I have:
In Canada, it (Cantonese) is the third most widely-spoken language. (First two are English & Francais)
In United States, it is also the third (First two English & Espanol)
In Socceroo (I like this name, although it's not formal), it is the second.
Also, in Southeast Asia, it has great influence among the residents
And in the aspect of Culture, Cantonese is more like the ancient Chinese, which reserved many characters, phrases, grammar structure from the ancient Chinese (Wenyanwen).
But one more thing, don't mix Mandarin and Beijing dialect by mistake, also Cantonse and other dialects similar to Cantonese.
Recently, the Chinese govt. is promoting Mandarin around the country. But has side effects is that many dielects are prone to extinct (Young generations only speaks Mandarin, forget their own dialects).