Good question. IMO, a good test kit is one that is accurate and precise.
IMO, accuracy to me means that when the measure says 550 ppm, it is truely 550 ppm. I found that my red sea kit is giving me 550 ppm. Then I bought a Salifert kit, and tested the water 15 minutes later and got 460 ppm.
IMO, precision for test kits means that the measurment increment is small. With red sea's calcium kit, it is a titration kit, and each drop means 50 ppm. So, you will only ever come up with something like 400, 450, 500, etc. With my new salifert kit, the increment is 12 ppm, so you can get 400, 412, 424, etc.
So you can see that you will get a more precise answer with the salifert kit. However, I am only assuming the 460 ppm salifert measure is more accurate than the 550 ppm red sea measure.
cheers,
kris