I'm French born Chinese and learnt Putonghua at the university in France. I was in HK 6 years ago. At that time, there was no putonghua in the MTR and when I spoke Putonghua, few HK people understood me and barely took attention to me if not despised me because they thought I was from mainland. I came back in Hong Kong in 2004 for holidays, and was quite surprised to hear Putonghua in the MTR. Now, I'm back in HK again for 6 months already: I realise that more and more people speak Putonghua and they don't despise mainlanders (or less than before). Maybe because most mainlanders in shops or restaurants are tourists that bring money to HK. The point is that I think Putonghua has a bigger place now in HK, and even bigger in the future. Though, I advise you to learn Cantonese for the everyday life because many HK people still don't speak Putonghua and consider it as a foreign language, so they will feel more comfortable (and maybe more helpful) while speaking Cantonese. And later Putonghua if you like learning languages.