I have tried both API and Salifert against each other, and they matched.
Maybe luck, but it was within the error of the test.
I stuck with API because the test is really simple, thus I test more often.
I do shake the reagents a lot every time before using, and wash vials + tops carefully.
From chem class, things like that can make a big difference.
Maybe someday I will drop the coin and go electronic.
Although I hear you have to calibrate all the time, so not sure if
it really ends up easier or better. Other opinions??