First, positions are different. Mandarin is a common national language, and is a legal formal occasion communication term; And Guangdong dialect (also call " Cantonese ") is one kind of the dialect of Chinese. Secondly, scopes of application are different. mandarin can pass through the whole country , Cantonese only use in Guangdong, most areas of Guangxi and Hong Kong, Macao. Thirdly, the language is different. The respects including the pronunciation, vocabulary, grammar, etc..

And as i am a Mandarin speaker, and l ive in Guangdong ,maybe i can say something about this question. For me, this is totally two languages, Cantonese's pronounciation is totally different from Mandarin, Cantonese contains much of ancient Chinese's struction, so maybe you need to know many ancient potery from China, you will be easy to learn this language. If you just interested in China, you'd better learn Mandarin, because it is more useful and popular now.

Mandarin is chinese tongue language, cantonese is a dialect,it is just spoken in some places