Kefir has more diverse bacterial and eukaryotic strains and is fermented longer at room temperature compared to yogurt.

“so I prefer kefir to yogurt and if people are not careful about the way they're making yogurt you can heat the yogurt too much and end up with a pasteurized essentially dairy product for what it's worth so if you're going to do fermented dairy products I prefer kefir because of the different an increased number of strains to make kefir bacterial and actually eukaryotic strains to make kefir and the longer room temperature fermentation process versus a shorter higher temperature process with yogurt”