You don't need both.
Which one to use is up to you. There are many good systems with both.
Calcium reactor is a bit more expensive to start with and it takes about 30 minutes of learning to figure out how they work and to tune them. They do offer calcium, carbonate, magnesium, potassium, strontium and a handful of other elements that were used for building the coral skeletons that you are now melting. If you have a finely tuned reactor, it will not move your PH, but it can drop your PH about .1, or so, if you are using too much CO2. All of the input in in balance. It is hard to crash or overdose since the output of a CaRx is really just to maintain and it can take weeks to raise levels too high. I just look every few days to make sure that the CO2 bubbles are bubbling and there is output from the reactor and then leave it alone for months at a time.
Dosing can be cheaper and easier to start with. The input is not balanced and you control each of the two or three part on their own. You can overdose a tank in minutes if a pump goes out of whack. You do not need to buy any fancy stuff to do this and Dr. Holmes-Farley has recipes using simple driveway ice melter and baking soda. You can do this on your own by measuring and pouring in your tank, or you can buy automatic dosing systems.
With either, you still have to test. I test alk about twice a week and rarely check calcium or magnesium (maybe every months) since they come in balance.