http://www.nikkei.com/article/DGXBZO37629750Y1A221C1000000/ In conversational Japanese, にほん is more frequently used. I use にほん、but that may not be the case for everyone. However, there's generally more of an inclination to use にほん. It's quite interesting though. I hope someone else can explain why Nippon is also used.

You can see the next page. https://en.wikipedia.org/wiki/Japan Etymology Main article: Names of Japan The English word Japan possibly derives from the early Mandarin Chinese or Wu Chinese pronunciation of the Japanese name, 日本, which in Japanese is pronounced Nippon About this sound listen (help•info) or Nihon About this sound listen (help•info). Japanese people refer to themselves as Nihonjin (日本人?) and to their language as Nihongo (日本語?). From the Meiji Restoration until the end of World War II, the full title of Japan was Dai Nippon Teikoku (大日本帝國?), meaning "the Empire of Great Japan". Today the name Nippon-koku or Nihon-koku (日本国?) is used as a formal modern-day equivalent; countries like Japan whose long form does not contain a descriptive designation are generally given a name appended by the character koku (国?), meaning "country", "nation" or "state". The character nichi (日?) means "sun" or "day"; hon (本?) means "base" or "origin". The compound means "origin of the sun" or "sunrise" (from a Chinese point of view, the sun rises from Japan); it is a source for the popular Western description of Japan as the "Land of the Rising Sun". Before Nihon came into official use, Japan was known as Wa (倭?) or Wakoku (倭国?). The English word for Japan came to the West via early trade routes. The Old Mandarin or possibly early Wu Chinese (吳語) pronunciation of Japan was recorded by Marco Polo as Cipangu. In modern Shanghainese, a Wu dialect, the pronunciation of characters 日本 'Japan' is Zeppen [zəʔpən]. The old Malay word for Japan, Jepang, was borrowed from a southern coastal Chinese dialect, probably Fukienese or Ningpo, and this Malay word was encountered by Portuguese traders in Malacca in the 16th century. Portuguese traders were the first to bring the word to Europe. An early record of the word in English is in a 1565 letter, spelled Giapan.

訂正<ていせい>correction usefull→useful 回答<かいとう>answer I don't understand why you ask us like that. When you are asked "the United States of America X the U.S.A. X the USA X U.S.A. X USA X the States X the U.S. X the US X U.S. X US X America X Uncle Sam X Washington What is useful in the United States of America/the U.S.A./the USA/U.S.A./USA/the States/the U.S./the US/ U.S./ US/America/Uncle Sam/Washington? ", what would you answer? If your question is "Why do you have two words/forms "Nihon" and "Nippon?" to express "Japan" in Japanese, I can answer like below. 日本＝nit＋pon(both of them are pronunciation of Chinese language) ↓ nippon［nippoɴ］→niffon［niɸɸoɴ］※http://www.italki.com/entry/554151 ↓　　　 ↓ nippon nifon［niɸoɴ］ ↓　　　 ↓ nippon nihon［nihoɴ］