Of course Cantonese. Cantonese is mother tongue in GZ.
If you wanna  communicate with the native people in GZ, you can learn Cantonese.
but now many other city's people come to GZ to living or working, so many people also can speak Mandarin in GZ.

I think pinyin is very useful for you to study Mandarin. Pinyin can improve your pronounce and you can use pinyin to typewrite the Mandarin or Cantonese in your PC, you can msn with Chinese easy. pinyin is good for you. I think more practice is very important for study language. Cantonese and Mandarin, I think you can see chinese movie and soap opera , chinese news or talk show .