Ca reactor will add Ca, Alk, Mg (consider supplementing ZEO Mag in the reactor) and trace elements. Draw back is the residual CO2 can cause low pH.
Kalkwasser will add Ca, Alk, percipitates Phosphates, and raise pH. Drawback is no Mg and no trace addition which are not big issues due to w/c and supplementing Mg (Mg Sulfate & Mg Chloride solution).
Many people use both, they do supplement each other. Kalk can be added very easy by using 2 tsp p/ gal of RO/DI. Shake and cover, let settle overnight. Pour the top 3/4 gal into a Kent Aqua Doser. Try to avoid the residual solids, though no harm would happen if the solids do get in there. Sometimes Kalk reactors may stir up the solids, thus I prefer the manual mixing and drip at night method. Cheap and effective!
If you had to pick one of the two methods, I would go with Kalk.
I just use aKalk drip and supplement DIY two part when needed. I have excess indoor CO2, so I didn't want to go the Calcium reactor route.
By the way, Randy Holmes-Farley does not use a Ca reactor, only Kalk and Two Part w/ Mg supplementation.