Yoghurt is a good source of probiotic bacteria, requiring weekly maintenance, depending on the culture chosen. It is generally a spoonable consistency and may be mild or tart in flavour. Yoghurt can be used in a variety of recipes.
Milk kefir is a great source of probiotic bacteria and yeast and requires daily maintenance if using milk kefir grains (not Mad Millie kefir uses a culture not grains). It is generally sourer in flavour and of pourable consistency. There are countless milk kefir recipes.
Yoghurt and kefir have different strains of bacteria so provide different benefits and different tastes and flavours.