Last week, 260 contracts filed on the SEC’s EDGAR system include one or more references to 50%. By contrast, only 85 used the word half, and mostly in contexts where one couldn’t have used 50%, as in references to “half-time basis” and “seven and one-half percent.”
I prefer half over 50%. Saying “50% of the shares” rather than “half the shares” is like saying “I ate 1.0 apples” rather than “I ate an apple.”
But I wouldn’t take this logic any further. For instance, I wouldn’t recommend that you use one-quarter rather than 25%. One-quarter is much more cumbersome than half.
Incidentally, Garner’s Modern American Usage recommends omitting whenver possible the preposition of after half, but notes that “when a pronoun follows, the of is typically needed,” as in “half of them are.”