Which of these two theories is more accepted or has more support in the field of ethics?
It seems that Kantian ethics holds too stringent of rules and the the Universal Law of Formulation is difficult to apply to all situations.
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First of all, there is a category error here, comparing utilitarianism -- an entire class of related forms of ethics, to Kantianism -- a very specific theory, is not possible.
If one chose utility functions in such a way that autonomy was a highly valued form of pleasure and utilities topped out at a given level, you could pretty much achieve a form of utilitarianism that consistently gave the same results as Kant. So then how can you decide between these isomorphic but independent theories?
But even then, the two have rather different orientations. Kant is not intended to work out the solution to very specific problems. The notion of contingent duties means that it will very seldom actually give you a direct answer to a fully expressed question. At some point, you will introduce a detail that causes some aspect of some duty involved to be contingent, and then in deference to your own autonomy, Kant will not tell you precisely what to do.
For general questions like murder or lying about which humans already have strong intuition anyway, Kant gives a reason to trust our existing human instincts by putting them into a general framework in which most folks can see the value.
Utilitarianism has very much the opposite focus, it is almost impossible to apply to general questions without details, because you cannot predict the suffering of someone without knowing pretty specifically how they are going to be treated. And even then, you cannot propose a way of combining different kinds of suffering without a strong cultural basis for differentiating values: do we find mental anguish to be pain, or the fear of change? Which one should we weighted more? Different traditions would give rather different answers there.
So each is terribly weak at different kinds of questions, and where each is best, that one is generally going to give you back a clarified reflection of what you, yourself, put in.
So in practice, the value in these systems is not really in determining morality, but in grounding agreements and opening negotiations. Neither is generally better at this.
This is a complex issue. I Just make a little sketch, since I am not certain what the core of your investigation is concerned with.
If one asks in terms of moralitate (morality, as manifested by individuals), and sittlichkeit (formal and supposedly higher guidance), somehow, theoretical ethics, is very vexed. Since what is in power, in government practices, in the international and domestic spheres, is not in lock step with the mores of the countries involved. So what does "ethics" refer to, what the largest number of Professors of Philosophy say? What the most popular Public Intellectual is currently selling?
If one says that "interest" (Intrest Poltics, Realism) corresponds to Utilitarianism, i.e., one removes justice (steming from a perpetual source, or a God), and what is left over is Interest, than Utilitarianism looks a lot like Positive Law, whatever the current law is (i.e., relativism). Yet, nobody of influence promotes themselves like that in practice.
Someone can have an "interest" in Justice, for instance in the sense that the Qaddafi killing was normatively justified in the name of "human rights". Yet this is often called "moralism". It could equally be understood, cynically, as concealed or rationalized Interest.
It's not really that a universal law is hard to apply, because "equality" is equally a universal principle, that must be interpreted into individual (specifically purpose) laws and situations, but rather that that Kant's specific notions are not as ideologically attractive as others, such as the rising power of "inclusivity". Behind Kant's principle is a claim that one should act as the founder, almost as father of the country, and treat oneself and everyone else as children. In the positive sense of taking responsibility for one's acts, but, largely, the notion that the human being is basically to be understood in terms of rationality is no longer believed in, if, in weaker cases, only because of the notion of "bounded rationality", the factual impossibility of being sufficiently well informed. and, in stronger terms, the notion that the human is essentially dominated by biases rules Kant's notions out.