Of course it depends of the person, but generally spoken language is much less formal than written language. People have higher standards for grammar, usage, etc when they are reading than when they are listening. Additionally, slang is far more accepted in spoken language than in written language. To get more technical about the subject, in English there are primarily two types of words: words that are romantic in origin and words that are Germanic in origin. Romantic words, which come from Latin, are much more common in written English than they are in spoken English. This is because usually we associate both Latin words and written language with a more formal or academic environment. Germanic words are more common in spoken language. We usually think of these words as being more informal.