A kalkreactor is a reactor filled with super saturated kalkwater, extra kalk, and a stirrer of some sort to keep the extra kalk from setteling on the bottom. The kalkwater is then dosed in to the aquarium just like you would normally with kalk. The purpose of the reactor is to automate the kalk process so you dont have to do as much work constantly mixxing up new batches. You only have to refill the reactor whenever it gets low, and make sure that the fresh water feed to the reactor remains full.
A calcium reactor is entirely different. It is a reactor filled with dead coral skeletons. A Co2 tank w/regulator is attached to the calcium reactor and it constantly pumps a small trickle of CO2 into it. The CO2 lowers the PH inside the reactor which dissolves the dead coral skeletons and puts calcium and alkalinity back into the water. There is also a small feed pump that delivers tank water to the reactor, and a small drip line that comes off the reactor and delivers the calcium/alk enriched water back into the system. Although this is a very automated dosing system and requires less work then dealing with kalk, it still requires constant testing like any other method.
Dosing kalk raises the tank PH. Using a calcium reactor lowers PH. It is common for people who use Calcium reactors to also dose kalk to counteract the "chronicaly low" ph problems associated with using calcium reactors.
In the end however alot of it depends on how much dosing you really need to do. Calcium reactors are meant for systems with heavier alk/calc demands, and it is hard to set them to deliver only a small amount. Kalkwasser and two parts are much easier to use on small systems, or systems without a lot of growth. If you find yourself unable to keep up with your alk/calc demands using Kalk and A+B solution, then you might want to consider upgrading to a calcium reactor.
Peace,
Jesse
