To decide upon a style it is important to work out whether the letter is to inform, persuade or entertain.
When writing to inform it is necessary to keep the language and style clear and to the point. This allows the reader to pick the information up quickly and without ambiguity.
When writing to persuade the style and language should be more inviting, encouraging the reader to see from the writer’s point of view.
If the writing is meant to entertain then it should be written in a language and style that is friendly and warm.