I just want to post a few quick thoughts for the night on the topic of voice in narration versus narration in dialogue. The opinions I state here may change with time as I learn more of the art of writing, but these are my feelings for now.
Unless there is a specific reason to give the narrator an accent or flaws in his speech, the narrator’s grammar and speaking should be perfect. To my mind, this establishes a baseline against which the characters’ voices can be heard. It also establishes the author’s expertise and shows that the author knows what he/she is doing with regards to the language. If the narrator’s speech is perfect, then any accents or flaws or flourishes in the characters’ speech can be seen more distinctly. I believe the narrator’s voice (unless there is a specific reason for otherwise) should be simple, clear, and free from anything that might draw away the reader’s attention from the storyline. When I read, I am very focused on understanding the interaction of the characters, their solutions to problems and situations in the story, how the plot is developing and so forth. If the narration is overly ornate or full of irregularities or intricate devices and I have to resort to a dictionary to understand what is happening, my reading is interrupted, my concentration is broken, and my enjoyment of the story is diminished.
On the other hand, once the narrative baseline has been established, I can play with the character’s speaking styles in any number of ways and they will (hopefully) be more obvious because they stand in contrast to the narration. Again, unless a specific reason exists for doing otherwise, their voices should be clear and easy to understand. An example of this might be the case of a scientist, whose complex mind is shown by his use of scientific jargon and overly complex sentences. An example at the other end of the spectrum might be an illiterate bumpkin, who uses very simple words, bad grammar, and frequent vulgarities. Of course, you might also show that there is a hidden side to the character by their speech patterns. Suppose that the bumpkin mentioned above every now and then showed that there was more to him than met the eye by using scientific jargon or by having exceptional command of English.
I also like to toy with being able to distinguish characters by speech patterns and jargon. A sailor might use a lot of nautical terminology and Navy slang, but the reader might also be able to identify him through repeated use of a certain term. I used to have a friend that used the word “Whatever!” very frequently. His speech would be very easy to pick out in a dialogue of several people, even if the narrator did not state specifically who was speaking.