I have found that milk kefir has more benefits than water kefir. I personally have noticed that milk kefir creates significant health changes that water kefir does not. There are benefits in both water and milk kefir, but milk kefir has a larger amount of beneficial bacteria and is, by far, the most beneficial of the two.
Check out this post for more info:
https://www.culturedfoodlife.com/lesson/which-is-better-water-kefir-or-milk-kefir/