I am wondering, because the two languages seem similar in culture and exchange of characters. I am interested in Japanese but mandarin seems more useful as China is a rising power and its language of a billion people. If I learn one of them will it make the other easier?

As I'm sure you've figured out from other answers or your own knowledge, Japanese and Chinese share kanji. A Chinese person could attempt to read a Japanese newspaper and gather a very vague idea of what's going on. However, the pronunciation and usage of those characters are radically different.

Most kanji (Japanese name for them) are based on some pictoral aspect. In Japanese, a single kanji may carry 2 or more readings that can change depending on what it is surrounded by. They also can have completely new readings when combined in a certain word (up "ue" + hand "te" = skillful "jouzu").

In Chinese, however, this is the only alphabet they have, so they are mostly phonetic. A single character will usually only be read one or maybe two ways. Plus, since this is their only language, most characters have been simplified in one way or another, and may not be recognizable to one who reads Japanese kanji (just as you might not be able to read calligraphy even though you know the characters).

Besides this shared written language (which is totally different), there is really nothing all that similar between Chinese and Japanese. There are some traceable etymological roots where you can figure out how a few words may have been brought over from China, but really there is no hope of the two understanding each other.

Japanese is an alphabetized language, meaning all words are formed with a set of phonetic alphabets. There are two sets of alphabets (hiragana and katakana). The two sets are identical phonetically but written differently with the latter being used to sound out more modern English words, for example. 'a-re-be-ta' = elevator.

The 'twist' comes in that many words can be written either in hiragana, or Kanji, which looks like Chinese characters. For example, the word water = 'mizu' can be written as two hiragana alphabets (mi-zu) or one Kanji.

So to answer your question, if you know how to READ Chinese you would be able to identify a majority of Japanese Kanjis but you would not be able to pronounce them. Moreover, the grammar are completely different and you would need to know the usage of hiraganas, so knowing all Kanjis would not suffice to actually know the language. So knowing one DOES help you with the other, but only in reading comprehension and not helpful at all in verbal communication or grammar.

To complicate matters, the Japanese Kanjis are taken from the Traditional method of writing Chinese Kanjis (the system used in HongKong and Taiwan). China is mostly populated with the Simplified version today.

Hope that helps.

two languages are totally different
but
Japanese is harder than Chinese
and
therefore Japanese people learn Chinese better
than
Chinese people learn Japanese
therefore
for a European it's better to learn Chinese
if
you only aim at learing one language
however
if you want two start with Japanese

japanese language and chinese language are totally different language. not similar at all. nobody can understand chinese without lesson in japan. chinese is same as latin and russian to us :)

grammar
japanese = subject + object + verb
chinese = subject + verb + object

pronunciation
japanese = really simple. no irregular reading rules like phonics
chinese = each characters have 4 different tones but same sound

characters and letters
japanese = there're 1 character system and 2 letter systems (using complicated characters) use all systems in one sentence.
chinese = only 1 writing system (using simplified characters now)

so there is no single profit if you learn one of them.