Tyler Cowen, quoted in this overview of the economics of meat by Mark Bittman, says that while an environmentally-aware meat eater should eat more pork than beef, "it is better for animal welfare to eat cows rather than pigs".
In reality, the best choice from both points of view is to eat less meat and better meat: happier animals are tastier animals, and there are hundreds of millions of Americans who’ve never eaten a grass-fed steak or lovingly-raised pork. Once they do, they’re much more likely to want to eat less meat of higher quality.
Another important development, which I know that Cowen would enthusiastically agree with, would be for Americans to eat more of the cheaper bits of the animal, especially things like brains and tongue and sweetbreads.
In both cases, there’s a vicious circle: there’s no supply because there’s no demand, and there’s no demand because there’s no supply. I’m hoping that supermarkets like Whole Foods might kick-start a more virtuous meat-eating culture.