Mandarin, by far :)

Mandarin

I prefer Mandarin. 

Personally? Cantonese ... But I get more done with manderin.

I have chosen Mandarin and, if asked, would recommend others choose Mandarin.

First, there are over 15 times as many Mandarin speakers as Cantonese speakers (according to Wikipedia, at least).  So, by choosing Mandarin, it is far more likely that you'll find people with whom you can talk.

Second, in my own experience, people whose first language is Cantonese will often also speak Mandarin.  This furthers the likelihood that you'll find people with whom you can talk Mandarin.

Third, based on my own experience in visiting Hong Kong, you're fully able to get by speaking either Mandarin or English.

Fourth, there are far more resources available for learning Mandarin.  A simple search on italki.com shows the following teachers:
Mandarin: 62 Professional Teachers and 210 Community Tutors
Cantonese: 4 Professional Teachers and 23 Community Tutors

These are all sweeping statements, based on my own experience.  Based on that experience, I have chosen to learn Mandarin.