You should consider comlexity, maintenance, and cost.
Two-part is fairly simple. I've never used two-part, but if you have to do something everyday, I would consider that high maintenance. Aren't the monthly costs higher with two-part?
A kalk stirrer gets loaded once a week and the Ca reactor twice a year. That's low maintenance. You already have them both, so there is no significant cost increase. However, the Ca reactor adds some complexity to the system and another point of failure.