Each new font is a distraction, forcing the reader to reacclimatize to the letter forms to continue through your message. For business correspondence, letters and proposals, keep your font selection simple. Stay within the same font family. Computers make it easy to experiment and achieve the special look you want. Lower Case Letters are Easier to Read.
People read by recognizing word shapes.
Capital letters are all basically square and are therefore less readable. When preparing long blocks of copy, always use lower case letters. All upper case letters can be used for headlines or for titling sections as long as they contain just a few words. At times, you may want to use upper case letters to add emphasis to a word or short phrase. In those instances, use the small caps feature of your program to decrease the font size slightly.