Nutritionally, is it better or worse to introduce cow's milk at 12 months if the baby is still breastfeeding?

No effect: I don't believe it is either good or bad for most babies, whether cow's milk is started at one year of not, if breastfeeding is continued. Breastmilk can't be beat easily as a perfect food but it is not enough alone and a nurtritious varied diet is best at this age. However breastmilk is an adequate replacement for cow's milk if still breastfeeding.