A ditherer is a person who keeps being unable to make a final decision — someone who dithers. The word names the person, not the action. It is informal, and it is usually a little critical, though not a cruel word: it paints someone who fusses, delays, and cannot settle. You might call a politician a ditherer if they will not choose a side, or a friend a ditherer if they take twenty minutes to pick a sandwich. Though actually, the same person may think carefully rather than badly; ditherer still says, from the outside, that the deciding is taking too long.