kinda a tough poll there. they kinda do different things. i would say start with a kalk reactor first. if this is unable to keep the levels up than add a Ca reactor. the Ca reactor is great at raising levels, while the kalk reactor is good at keeping levels. it also supplies the much needed alk to the system. i think they go hand in hand and should be used together.
Geoff's right. Kalk should be sufficient unless you have lots of SPS corals.
However...keep these tid-bits in mind:
Kalkwasser is great. Once your Ca and Alk levels are where you want them, start dosing Kalk, and they should stay there (unless you have lots of SPSs or calcium hogs like Clams, etc). One downfall though is that over time, Kalk use can cause your Magnesium levels to deplete. Frequent water changes should make this a null issue though.
Calcium reactors are MUCH easier. ONce set up, you barely ever touch 'em. Maybe every 6 months, you add new media and/or replace your CO2 canister. But once dialed in, that's it. Also, Calcium reactors (as a side affect) also add back some trace elements as well. But again, frequent water changes will do the same thing.
Either way, if you get ONE in line (Alk or Ca), the other should fall in to place...so either way can work.