A calcium reactor can keep up better with a SPS dominate system as it uses the tank water and CO2 to break down dead coral to make Alk and Cal. With its use, you can experience a drop in your Ph, but not always. Depends on how you plumb it and the type of reactor.
A kalk reactor uses makeup RO water to add diluted lime (which just happens to have exactly equal amounts of Alk and Cal) to your system. With its use, you raise your Ph and are limited in how much you can add by the Ph increase and how much makeup water you go through in a day. One advantage of using lime water is that it also precipitates phosphates. Downside is it will not keep up with an average/heavy SPS tank need.
Some use both to compensate for the lowering/raising of the Ph.