There are many choices when it comes to sugar - what you decide to use depends on your desired result.
Rapadura is the least refined sugar. It's produced by pressing the pure juice from the sugar cane, then cooking at low temps to evaporate the water off. The end product still includes the molasses, which holds all of the vitamins, minerals, and nutrients. Because of this, it has a stronger molasses-like flavor. Rapadura is slower to metabolize, unlike more refined sugars, making it a better choice health-wise.
Demerara, turbinado and muscovado are more processed than rapadura. The molasses has been removed during crystal formation. Then some is added back in for nutrients and color. Chemicals are sometimes used in the process. These sugars have a bit of a molasses-like flavor as well.
White sugar is the most processed sugar. Chemicals are used in the refining process, and no nutrients remain. White sugar is fairly flavorless.
Brown sugar is refined white sugar to which molasses has been added.
We conducted experiments using several types of sugar for our water kefir: white sugar, demerara, sun-dried cane sugar, muscovado, molasses sugar, and rapadura. They all produced well for awhile, but over time the grains began to deteriorate with all but the rapadura and white sugar. The grains lost their clarity & luster, and disintegrated a bit, like applesauce. Our conclusion was that the rapadura seemed to work best for grain health, and white sugar- believe it or not- came in second (this is our opinion, not a scientifically proven fact) ;)
We use rapadura for our water kefir grain production because it contains the nutrients & minerals that the grains love. It keeps them happier, healthier, and productive- BUT rapadura produces a strong-flavored kefir that is off-putting to some (including us;). Stronger flavors are helpful during second fermentation to balance out the taste.
When we make water kefir for our families, or for customers to sample, we use white sugar. Although it's least desirable as far as nutrition is concerned, the mildness of the white sugar allows the kefir to be flavored more easily, and is appealing to more people.
If you prefer to use white sugar, it's a good idea to put your grains back into rapadura occasionally to give it a boost. The rapadura can also be mixed with the white sugar, kind of like a nutritional supplement.
So, taking all of this into consideration - the choice is yours! Take good care of those little babies, and they'll do the same for you :)