I think the horse nipples would be better because a horse needs more fatty milk for growth. Human milk has less fat than horse milk. If a centaur drank human milk the horse half might not grow properly. Also, Humans can consume horse milk.
Probably both. Horses can stand within moments of birth, humans cant. So its debatable if a centaur foal could stand and control the human half.
So a mix of both id guess. That way they get the nutritional value of both.
So a mix of both id guess. That way they get the nutritional value of both.