Kefir contains more probiotic strains and a higher concentration of probiotics than yogurt. Kefir typically contains around 40 billion probiotic organisms per half cup serving, while Greek yogurt contains around 60 million CFU per gram. The probiotics in kefir are also more likely to colonize the intestines long-term compared to the transient bacteria in yogurt. However, both kefir and yogurt provide health benefits by maintaining digestive health and balancing intestinal microflora.