I would make the decisions based upon if you are going to keep the course with heavy stony corals and how long you might be in the hobby. Nearly nobody that gets a CaRx, and takes the time to figure out how to set it up, regrets it - this says something if you are paying attention. Dosing is good too, just more variables into the mix.
I use a CaRx for a few different reasons - input in balanced, so I only test for alk. I will test for calcium every few months. I ignore magnesium, strontium. When melting natural media, you supplement all of the major and minor trace elements that the coral uptook when they once grew. You need to spend 10-15 minutes a day in the first week to figure out how they work, and how to tune them, but once you learn then you are golden. They work slowly and are only capable of maintaining, so rising or lowering is really really slow - they are nearly impossible to crash a tank with unless you stop paying attention for weeks.
I could totally make 2/3 part work... just choose otherwise.
I like Kalk too, but I cannot evaporate enough water to use kalk... just too much demand.
Cost? 2 part used to be cheap, and still can be. However, needing to use an Apex and DOS will get up there in cost. CaRx is never super cheap, but even a super-expensive one does not work better than a normal cost one... just different. A really reliable 2 part system with redundancies and stuff is not going to be any cheaper than a CaRx.
Ease of use? A CaRx will need an hour of time over a week, or so, to tune it and figure out how. If you cannot figure this out, then programming an Apex or doser is not going to work either. In all actuality, both require thought and brains or else you will fail, but we are not sending people to Mars, or anything. Dosing can crash a tank in minutes if a doser sticks on, so in this way, they are not easy if your alk goes to 20 or your calcium to 1000 and everything starts to precipitate everywhere.