I am not yet an expert in Chinese, so I like to cite this article which clearly shows a difference. Keep in mind that the author learned both languages at DLI. That is the Defense Language Institute. He studied Mandarin before Japanese, so he was an experienced language learner when he studied Japanese. He was put through an intensive course and he didn't find that to be particularly difficult. Then he spent over 7 years in Japan and still didn't feel comfortable with the language. And he was not the only one who felt that way.
Japanese is hard to get used to. People keep changing their speech so you don't get enough exposure to it. The writing system can write things in different ways, so again you are robbed of exposure. You always stop to think about the verb form even though the meaning doesn't change. Well, you don't always stop to think, and then you end up not saying it the way that you know is better.
Anway, I'm going to paste that article in here because I'm afraid that one day it will no longer be available through the link. Please read the whole article. It is quite interesting. So, here it is:
Chinese Mandarin is Easy
Comparing Its Difficulty with Japanese, German, and Spanish
by Mike Wright
"The biggest impediment to learning Mandarin seems to be fear--sometimes caused by the teachers. I've studied quite a few languages, and none of them were as easy for me as Mandarin."
What I came to believe is that Mandarin is pretty easy for native English speakers, while Japanese is one of the most difficult. Mandarin syntax is easy to teach using pattern drills. Furthermore, Mandarin sentence order is similar to English--but simpler, having no inflections (thus no irregularities) and with gender, number, tense, etc. being optional, whereas they are obligatory for most of the world's languages. The only difficult part of spoken Mandarin is the tone system. Even that isn't a big problem for practical use. I know that my tones have always been weak, but when I was using the language regularly, I had no problem communicating. What turned out to be more important was to adapt to the basic pronunciation and vocabulary used by the average Hokkien speaker when speaking Mandarin. Of course, I never got to go to China. I do remember how wonderful it was to run across a native of Beijing or Tianjin in Taiwan--it was so clear.
The biggest impediment to learning Mandarin seems to be fear--sometimes caused by the teachers. I've studied quite a few languages, and none of them were as easy for me as Mandarin.
Mandarin was my first serious language, after some Spanish and German in high school and college, and it was the easiest by far.
Comparison with Japanese
I didn't find Japanese too difficult while studying it at Defense Language Institute, but when I arrived in Japan, I found that I had a lot of trouble communicating. This was very different from my experience with Mandarin. When I arrived in Taiwan, I could pretty much discuss any topic. On the other hand, I spent a total of 7.5 years in Japan, much of it associating with people who spoke little or no English, yet I never felt confident in the language. It's not so much the syntax--the conjugation of verbs and adjectives is quite regular--but the way the language is used. In many respects, it seems to be as much a problem of culture as of language per se.
Japanese syntax, as usually taught in schools, covers about 25 percent of the syntax. Even Defense Language Institute probably wasn't able to cover more than about 60 percent. It's not that it's so difficult--there's just so much of it. Compared with conversational Mandarin, there seem to be many more common ways of expressing any particular idea. The Japanese seem to be more fond of synonyms, too, leading to the need for more vocabulary items. Japanese culture adds to the burden. The Japanese don't like to just come right out and make blunt statements. They talk around the subject. By comparison, Chinese speakers and English speakers are very much alike. They tend to be direct and precise. Although this is a matter of culture, it has a big impact on the ease or difficulty of learning the language of a particular culture.
So, I'd say that what made Japanese difficult for me (and for all of my fellow Defense Language Institute graduates) is that there seems to be so much of it, and that it's spoken by people who are living in the Japanese culture.
Many of my friends had similar experiences, including one who graduated from the Japanese course with a 98 average--the highest on record. He was quite angry when he arrived in Japan and found that he couldn't get around in the language as he had been able to do with Mandarin in Taiwan.
Comparison with German and Spanish
In comparison, German and Spanish are difficult because of inflection and gender. Although many people consider these languages easy because of the large number of English cognates, my personal experience is that vocabulary is nothing. You will pick up as much as you need--as you need it. The really tricky part is the syntax. If you don't have that down, no amount of vocabulary will save you.