As a native speaker of Cantonese in Hong Kong, I think Mandarin is a better choice overall. Cantonese is predominant in Macau and its neighbouring province - the Guangdong province. Besides these (as well as Hong Kong), actually the use of Cantonese is quite limited. Also, I actually think that people here can understand English (and Mandarin increasingly) well so communication is not really a big problem here if you don't understand Cantonese. Cantonese itself is quite difficult to understand if you are not native because it has its own unique features (which could be difficult, like complex ending particles and more tones) when comparing with Mandarin. Also written Cantonese, in my opinion, is not an usual thing to see - most write in standard Chinese, so Mandarin is a better alternative as it exposes you to the standard Chinese writing/characters/... as well.