For Calcium I'm a big fan of API. Very simple, cheap, and more then accurate enough for me. I just want to be in a rather wide range here and high resolution doesn't interest me at all. I see no reason to pay more money on a kit that will tell me I'm at 450 instead of around 440. Which for I don't know how many years I've maintained around the same calcium levels which can be seen by my 2010'ish API vs Salifert Calcium and Alk thread. They are still reading about the same and still about the same results between the two.
API: 440
Very easy to know when you've hit the end point
Just make sure to keep track of how many drops dropped...
Salifert: 450
Also, very easy to know when you've hit the end point at a slightly higher resolution at a higher cost for the kit
But at least you don't have to keep track of the number of drops like with API