They are all good natural probiotics and can all be used well in green smoothies or on their own and are generally just a matter of preference. However, there are some differences.
Rejuvelac, is made from sprouted fermented grains. It is one of the easier and least expensive foods to ferment. You just need a grain (wheat, rye, quinoa, buckwheat, tritcale, or other whole and unmilled grain) and water.
Water kefir requires water kefir grains and often calls for sugar water (although you can make it just fine without the sugar). It’s especially effective with plain coconut water.
Milk kefir requires milk kefir grains and milk. Raw is best. But if you can’t get that make sure it’s from cows untreated by hormones. And raw goat’s milk also makes great kefir. When fermentation occurs in in a dairy product, enzymes proliferate and milk protein (casein) becomes much more digestible. The enzyme lactase pre-digests lactose proteins, and several enzymes created by the fermentation process increase absorption of calcium and other minerals. Even people who are “lactose-intolerant” usually have no problem with cultured dairy products like kefir.