1) There are three main types of Mandarin spoken - Mainland Mandarin, Taiwanese Mandarin, and Singaporean-Malaysian Mandarin. Mainland Mandarin and Taiwanese Mandarin differ by about 15%.
2) There are also two types of Cantonese spoken - Guangzhou Cantonese and Hong Kong Cantonese, which are very similar differing by less than 10%.
3) While Chinese is considered one of the most difficult languages to learn due to its writing system and tones, Mandarin may be easier for some like Japanese speakers who already use many Chinese characters in their own writing system.