If you are going to live in Hong Kong learn Cantonese.
In 4/5 months of being here I've only heard people talking in mandarin once or twice, English is actually much more common. I recently had a friend visit me from the UK who speaks pretty decent Mandarin, and he tried to use this around HK but you could tell it was not appreciated a lot of the time. Also, regardless of whether they looked happy or annoyed to be talked to in Mandarin, locals seemed to understand Mandarin but not actually be able to talk back, instead replying in Cantonese (which my friend doesn't understand a word of).
Overall, I'd say 75% of his conversations involved the locals replying to his Mandarin with Cantonese, and looking slightly irritated to be talked to in Mandarin. 20% of the time the local HKers did not understand Mandarin, and the other 5% they understood and happily talked back in Mandarin! (Rough estimates of course... something like that).
As for difficulty, I don't think there's really any difference. Apparently English speakers are good at pronouncing Mandarin, and Mandarin has many many more learning resources than Cantonese. But, the fact Mandarin has less tones makes no difference in my opinion. You have to remember exactly how to say something to be understood no matter which of the two it is. The biggest factor will be motivation, and if you are in HK hearing Cantonese all day long you would find it a lot easier to find the determination to stick at Cantonese!