For my big system (300+g), hands down a calcium reactor, but I also add about a tablespoon per week of kalk through a kalk stirrer that is fed by my ATO, so all ATO water has kalk in it.
For my small (16g) coral QT, it's just kalk in the ATO for now, but I'll be tinkering with that once I assess the demands (which will of course change as I add and remove corals).
I quit the hobby around 2007 or so, and 2-part was not as big as it is now, so I'm just more comfortable with a calcium reactor now that I'm back in it. For a smaller tank I can see how 2-part (or 3-part) makes sense.