Hope this will help:
From a % speakers/opportunity to practice perspective:
In the world, 12.44% of the total population speaks Mandarin (obviously heavily skewed by China's massive population, but still 12.44% nonetheless), compared to 4.83% native English speakers, and a measly .89% Cantonese speakers. (http://en.wikipedia.org/wiki/List_of_languages_by_number_of_native_speakers)
In China, more than half of the population of 1.4 billion (53%) speaks Mandarin. And 66% of city residents speak Mandarin.
In Taiwan, heavy majority Mandarin speakers (don't have %), some Hakka, some Taiwanese, and virtually no Cantonese presence.
In typically majority-Cantonese population provinces:
Guang Zhou: percent of Cantonese speakers has dropped below 50%, and Mandarin is on the rise.
In Hong Kong, majority of residents speak English and Cantonese, and many understand Mandarin.
From a learning perspective:
- Mandarin Chinese has 4 tones
- Cantonese has 6 tones
It's generally much easier for students to learn Mandarin over Cantonese.
From a future perspective:
- The Chinese government is trying to sell Mandarin as the basis for other dialects, and uses that in most government jobs and mandates it as a base language in schools. That being said, you'll most likely find Mandarin to overwhelmingly overtake Cantonese soon. (http://yolearnchinese.com/archives/mandarin-or-cantonese)
So it depends on your goals...
If you know you want to work, live or study in Hong Kong, then learn Cantonese... But if your goal is to learn a language that you'll have many opportunities to use worldwide/in the business world, then Mandarin is the language to learn.