Chicken. Far more chickens die per amount of meat, and I suspect that they have worse lives, since it is probably easier to keep a whole bunch of chickens in a small space and cut off bits of them without anaesthesia. Brian Tomasik writes about this question here, although be warned that there are some pretty nasty pictures that you will have to scroll past to get to his estimate of the numbers.
I don't worry too much about their living conditions. We already eschew factory farmed meats, so the chicken is free range and the cattle is raised and butchered by people with a religious obligation to treat the animals relatively well. These are definitely things that are good to consider though.
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