As a native speaker of English: the thing about translation from Japanese verses to Japanese is that if do not know how to say something in Japanese, you cannot say it in Japanese. An attempt to simply use a dictionary to look up the English words will often bring you to misfortune because of general (but unwritten) differences between Japanese and English words.
Where as because of your much larger understanding for the English language, it is easier to take something in Japanese and express it in English. That is, you have the ability to use your built-in mental thesaurus to make sure the same point gets across. If (as a non-native speaker) you try to do that with Japanese chances are you'll make a mistake and the idea of your English phrase won't be properly expressed in Japanese.
In regards to written verses spoken, well there are two sides to every coin. While my instinct tell me to say that written is easier, there are certain advantages to translating spoken Japanese. It all depends on the situation, for instance if you are speaking to a close Japanese friend, it is definitely easier to translate him/her than the written word.
Why? Because they will help you to understand them, if you ask them chances are they'll repeat themselves, or re-phrase -- this is great because they have the built-in mental thesaurus for Japanese that you have for English. They'll make sure the proper point gets across.
On the other hand, translating the written word has the great of advantage of an unlimited amount of time to translate. There is no pressure to understand right at that moment, it's all at your own pace. Also it's much easier to pull out a dictionary and look a word up than it is when you're actually listening in real time to someone talk.
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