I have only ever used API, and I am getting ready to upgrade all my reef tests to salifert. IMO ATI is very hard to determine NO3 at low levels because the colors look so similar. I may be wrong about this but I think Salifert determines its level by the number of drops you use to change the liquids color. In other words, you get an exact number as opposed to guessing between 0 - 10 or so.
No Salifert is regent as well. Elos and Salifert show color when API reads zero.
I use all three and suspect I will end up using Elos or LaMotte and API in long run. API for daily testing when I am adjusting something in tank. Then use better kit for weekly or bi-weekly testing.
API is cheap and pretty good quality for the $$.
I don't think there is such a thing as an exact number that is 100% reliable with hobby test kits. I've only reached zero with Salifert one time is lots of years, most of the time it has a light pink. I am curious how many of you guys get 100% clear reading on Salifert or Elos?