Why not try both and see which one you prefer?

If you have to choose I think mandarin will get you further career-wise at this point, simply because it opens up the job market for companies that are doing heavy business with the mainland, and you're more likely to run into mandarin speakers doing business in other parts of Asia than Cantonese speakers. Admittedly, many of the companies that do business with China in Hong Kong are import/export based businesses now and can't necessarily pay a lot outside sales, but the service sector is growing and that's where people really need bilingual help.

That said, knowing Cantonese makes life in Hong Kong a lot easier, and once you know ONE dialect it isn't too difficult to pick up the other. And you can always try to learn cantonese from our sister product Popup Cantonese to pick up the basics and see how it goes.