I'm really interested in this subject myself. I've been brewing Kombucha for nearly two years and started on Kefir about a year ago, tried the milky kind (in non dairy milk), but I didn't stick with it and now brew Water Kefir.
I've been thinking about what strains of pro-biotics are in both drinks, and if I should be worried about the amount of sugar in them. After plenty of experimentation, I now make both beverages on the sweet side, so I'm definitely consuming some sugar from them, which I'm not that happy about. But, I don't feel any ill effects, and I do seem to resist catching the random ailments that strike my housemates, so I'm hanging on to the idea that these fun fermenty beverages are doing me some good. Anyway, I haven't shed any light, but just wanted to register my interest. :)