This may be the second most discussed question about Martinis. As noted, my preference is a twist; when I drink Bombay. To me the lemon flavors compliment the herbs of Bombay while the saltiness of the olives seems to overpower the subtleties of the herbal notes.
However, if my barkeep of the moment doesn’t have a herbal noted Gin available, such as Nolet’s or Lavender, then I will ask for olives. They are a better contrast to those floral flavors.