The memo: format--typeface
For the traditional legal memo, choose a serifed typeface for the main text. Serifed typefaces are those with the little extensions--serifs--at the ends of the strokes. Times New Roman, Garamond, and Palatino are serifed typefaces. Generally, serifed typefaces look more formal and are often considered more professional. Historically, they have been proven to be more readable.
Sans-serifed typefaces have no serifs and tend to look more modern or trendy. They include Arial, Futura, and Verdana. You can use sans-serif typefaces for headings--the contrast makes them stand out--but avoid them for the main text of a memo unless those you work for recommend them.
Better Legal Writing
Writing for the Legal Audience