With yogurt, you can ferment it for ~24hours and have a very very low Sugar
content. With kefir, it is a bit slower because of the temperature at which it is done, and doing it at the temp. of yogurt would affect the bacteria/yeast balance and create a very unpleasant product.
You can use specific strains with proven benefits to make yogurt, while you have to have faith in the strains in kefir grains.
Kefir is very easy to make though, as you don't have much to do then put the grains in some milk. I don't even filter it, I just remove the grains with a wooden spoon (some say wood is not ok but I properly clean it between use and have no problem. Wood is said to collect bacterias). They got very big and good looking, thanks to a good unhomogenized whole milk.
Kefir has beneficial yeasts which yogurt has not.
There are some polysaccharides made by a few strains of bacterias in kefir which exhibit potent anti-cancer activities. That substance is not well studied enough I believe. Eating the excess of grains seem to have benefits too. It is very high in kefiran, given that the substance is made to build the grains.
Once you have the grains they grow very fast, so it's just a single effort... If you can find fresh grains this is the best, I had dried grains in the past and well, they became very different than the grains I have now. Might be because of the milk I was using... It is said anyway to take many weeks becore you get a good quality with these.