The categorical imperative is a better guide for moral reasoning because it can be applied by the individual with a clear, understandable reverence for the morality of the whole community. Mill’s utilitarianism is also concerned with the whole, but the mechanism by which one can judge the worth of his or her actions is quite a leap.
When a utilitarian makes a moral decision about an action the primary concern is individual happiness. The agent must then consider the consequences of this action within the extent of his or her influence. While there is little argument we all want to be happy, we can not assess the moral worth of our actions on consequence. It is much too difficult to assimilate individual actions and consequences with the welfare of humanity as a whole. Conversely, Kant’s categorical imperative gives an individual a more reliable mechanism by which to judge his actions.
The test begins with intentions rather than consequences. It is easier to assess intentions than consequences, because intentions are not as specific to situation and reliant upon outcome. Instead, good intentions are simple moral intuitions that tend to produce good. If the whole of humanity shares good intentions, acting only upon those intentions which the individual would will for everyone produces the greatest good.