Lord knows I'm not a fan of Greenpeace (1, 2). But this article in the Guardian by their Executive Director is very good. Excerpt:
So the distinction diehard carnivores need to make is not between cattle and chickens or pigs but between intensively farmed animals – which depend heavily on grain or soya and are therefore in direct competition with humans for land – and those that predominantly eat grass or, in the case of pigs, heat-treated swill from our waste food mountains. This means no industrially raised chicken and pork, as it is nearly all intensively reared indoors on animal feed. Factory farming is not only unacceptably cruel, but unsustainably inefficient.