(이)랑 is what we say in everyday conversation, it's informal Korean, shows your interest in getting along with the listener. (와)과 is what we see in books, newspapers etc, we say it in an informal situation like when we give a speech in front of lots of people.

Hello ~ ^^ 랑 is speaking word like gonna. And.. 과 is speaking word or writing word. umm.. well.. 랑 is not polite writing or formal writing. 랑(이랑) is more friendly and more informally than 와(과). < This is right, too. ^^ 0

wow... it's too difficult question... even I'm native korean!!! I think difference is 'Nuance' matter. 랑(이랑) is more friendly and more informally than 와(과).