In what ways do Kant's categorical imperative and utilitarianism differ? How is that difference significant when both frameworks are presented with moral and ethical decisions?

The main difference is that Kant’s Categorical Imperative starts with the individual while Utilitarianism starts with the group.

Categorical imperative is Kant’s proposed way of evaluating our motivations for why we act. He talks about two kinds of imperatives (things that must be done). They are both based on the individual:

• Hypothetical imperatives, that have a specific end in mind, such as: To stop being hungry, I must eat something.
• Categorical imperatives, in which our actions are based on moral principals, and are an end in themselves because they come from our moral goodness. They are not to attain something but to emulate what you would want to consider as a universal law.

He says, “Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.” The way he puts it is complicated, but it is really a secular way of dealing with moral duty and ethical decisions.

The Church created moral duty using the motivation of our punishment with eternal damnation. The State imposes ethical duty with imprisonment and other punishments. Kant says that it’s not because of our fear of hell or punishment that the good person chooses his actions, it’s because he has figured it out that it is his duty to be good and do good things.
​
Possessing good will is one thing, but the reason we actually perform a good action is the result of our obligation to do the right thing. In other words, we ought to. So, it is a highly individual effort, focussing on the higher good of others as a result of our choosing the good action over the bad.

Utilitarianism starts with the group and then goes to the individual. It is about figuring out what is the greatest good for the greatest number and then basing our actions on that deduction.