Note: in this page, "animal" refers to any non-human animal. Unless otherwise noted, it excludes humans from its definition.
Is vegetarianism more moral than omnivorism?
1: Yes
2: They're about the same in terms of morality
3: Omnivorism is more moral than vegetarianism
What a classic start, lol! Of course, the complication here is that "morality" is an ambiguous term, it can mean different things. For instance, if we intend "morality" as in "whether one approves or disapproves of a behavior/situation", then that's completely subjective, and so morality can be anything. If we intend something a bit less subjective (i.e. a set of principles that can be applied to the level of a society, rather than being the subjective feelings of an individual), there are still various formulations of morality: utilitarian, egalitarian, Kantian, Ahimsa, non-aggression principle etc so the variety of possible answers decreases. A superficial (though not necessarily "invalid") interpretation may be that, since violence/killing/causing suffering is immoral (in most "moral" systems), then eating animals, or at the very least killing them for human consumption, is immoral. Another interpretation is that moral values do not apply to animals (neither as moral agents, nor moral patients). This interpretation, however, could mean that it's morally acceptable to torture or rape animals. It's a possible interpretation, in the sense that it's thinkable, indeed. List:
Morality in the sense of subjective feelings:
1: Vegetarianism is more moral than omnivorism
1.1: Opinion based on empathy rather than a set of formal principles.
1.1.1: It's easier to empathize with animals that look like humans: cows, chickens, pigs, rabbits, dogs, cats etc. they have two eyes, a mouth (or a beak, but that's similar enough to a mouth), usually two ears, a nose, and so on. It's easier to empathize with those compared to an ant or to a jellyfish, whose reactions to pain are much different than those of a human.
1.1.2: Empathizing equally with all animal species: including insects and all types of invertebrata (e.g. worms).
1.1.3: Empathizing with all animal species, not necessarily equally (the suffering of animals that more closely resemble humans may elicit more visceral reactions), but at least to the extent that one wishes their suffering to end.
1.2: "If you eat meat, you don't care about animal suffering. Therefore, eating meat is immoral" (deontological argument, which ignores the fact that even if everyone went vegetarian/vegan, wild animals will suffer due to predation in the wild, and for other reasons as well, of course)
2: They're about the same in terms of morality
2.1: "Our ancestors used to eat meat, it's natural" (appeal to nature argument)
2.2: "Even if everyone was vegetarian/vegan, animals will still suffer due to natural predation" (consequentialist argument)
2.3: "Predators eat other animals, so it's ok if humans do it as well. Humans are not separate from nature" (tu quoque argument)
3: Omnivorism is more moral than vegetarianism
4: Anything else that can be thought/felt, really.
Morality in the sense of widely applicable sets of principles (at least based on some axioms):
1: Animals can be moral patients (though not necessarily moral agents).
1.1: Therefore, it's immoral to eat them.
1.1.1: Humans (as a whole (Gestalt-like entity), not as in "each" nor even "most" individual humans) have a moral duty to protect animals from being predated.
1.1.2: Humans have no duty to protect animals from being predated, but that wouldn't be bad either.
1.1.3: Humans should not protect animals from predation (may or may not contain romanticization of "the natural order").
1.1.4: Humans should increase the percentage of animals who, once born, will not be killed, neither by a predator (animal) nor by a human, but humans should not try to protect all animals.
1.1.4.1: The ideal number of animals that should be protected from predation is that which is slightly below a "critical number" at or above which ecosystems may be destroyed.
2: Animals cannot be moral patients. (Sub-ideas coming soon)
Consequentialist vs deontological morality (more specific ideas coming soon):
1: Consequentialist morality: one regulates their diet depending on the consequences of such diet.
2: Deontological morality: eating an animal has its own moral status regardless of the broader consequences.
Intended goal/what types (if any) of suffering one tolerates animals being exposed to. Note: these attitude may have two "versions": one is pragmatic and the other is more principled. That is, the distinction of whether the animal suffering is tolerated due to principle ("it's ok in principle that the animals suffer for the following reasons") vs being pragmatic ("it's not ok in principle that the animals suffer, but due to pragmatic reasons, solving that situation is either non-feasible, or feasible but at unacceptably high costs and trade-offs").
1: Hedonistic/utilitaristic worldview: To minimize animal suffering as much as possible (ideally eliminating it).
2: To minimize animal suffering caused by humans. Animal suffering caused by other animals or natural causes (e.g. starvation, dehydration, natural disasers, predation, disease etc.) is ok.
2.1: Because it's nature. (appeal to nature argument).
2.2: Because it'd be hubris from humans to think that "we" (i.e. humans, understood as a collective) should interfere with nature. (appeal to nature + non-intervention).
3: Total indifference towards animal suffering.
4: Indifference towards animal suffering with some exceptions (same as "2" but one is also indifferent to at least some types of human-caused suffering):
4.1: Such as... being against raping, torturing or otherwise abusing (e.g. with electric collars) animals. Types of animal suffering tolerated: non-human caused suffering (e.g. starvation, dehydration, natural disasters, predation, disease etc.), and human caused suffering but only limited to farming.
4.1.1: Broad tolerance on farming practices, even cruel ones, as long the additional cruelty is done for economic/pragmatic purposes rather than gratuitous sadism.
4.1.2: More limited tolerance on farming practices, with varying degrees of opposition to cruel practices. One may want more regulations on the conditions of the animal in farming, even if one is not against killing them for human consumption.