Okay, i am going to University soon to learn Chinese but the problem is, which dialect, mandarin or cantonese? *if that's the right spelling*

I am going to do a double honours with international business but have heard that mandarin is the more business language of China is that true? The only problem is is that i eventually want to work in Hong Kong, but i have heard rumours that they mainly speak cantonese there, is this also true? In which case does the fact that the business language is mandarin apply in Hong Kong?

If this post makes any sense, can someone reply.