hmmm two-part (or even three part) vs calcium reactor, geez why not just start a war... seriously I have used both so here goes my opinion..
both are very good and grow coral well! But in my mind there is a point where a reactor is worth the changeover...
1) salt stability.. two part will cause a tank to increase salinity. Now my tank often suffers from salt decreasing
2) trace elements.. reactor relies on whatever trace element is in the media, two part whatever is in the mix...
3) overhead cost is lower once the reactor is purchased. I ran my first reactor without a CO2 controller so it is possible to cut a bit there but quickly bought a CO2 controller (run now APEX)
4) while amount of Ca, Carbonate, and Magnesium can be adjusted by flow (increase the flow yields more of each), there is a break point where the water just is not in saturation.
5) lastly big one is fear of PH swings with a Ca Reactor, never had that problem but had a problem when my drip system for two dumped 1liter of A part into system (90 gal) in 24 hours.
All in all, I like the Ca reactor for larger systems or for less daily maintenance. But neither protocols are "no maintenance."
Lately, I have been considering putting a Kalk Reactor back online to provide some PH buffer at night but let the CA Reactor take the bulk of the CA/Carbonate/Mag work.
Lastly, just like all two-parts are not equal so not all CA Reactor medias are not equal. Some need a Mag supplement whereas others have more Mag to start with.
Short answer time: I DIY'ed my first reactor and now have a Koralin that I picked up at a good price and would not likely go back to two-part except for a small all-in-one system.