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If you have the energy read the instructions online to get a feeling for what each test involves. Detecting color intensity or color differences is involved in all test kits.
NO3 - Salifert and Red Sea give a colorless solution for zero nitrate. API does not. Both Salifert and Red Sea can detect NO3 below 0.5 ppm. Red Sea is cheaper but just a little more complicated.
PO4 - Salifert is colorless at 0 ppm and can detect down to around 0.03 ppm. Hanna ULR Checker claim to fame is that it provides a result far more consistent than the human eye with a color comparator and detects color below the level that most human eye sight can. With this Checker, PO4 is detectable below 0.03 ppm but with the inherent variation in the method, detection below 0.03 is a rough approximation that can be improved by running tests in triplicate. I have switched because I wanted to avoid color chart use.
Alkalinity - there isn’t anything fancy about titrations but the differences in color change at the endpoint can be very important depending on how well you see color differences. I can detect a Salifert endpoint more easily than an API endpoint. Salifert uses a syringe that delivers smaller drops than API which means it can be easier to determine the true endpoint. Recently, I switched to the Hanna Alkalinity Checker. It gives the same result as APi but there is no titrating annd no judging color change. Results are likely to be more consistent with the Checker.
Ca - I found the API test to be comparable to Salifert but much cheaper, though I use a syringe with the API test in place of the supplied dropper bottle to improve the consistency. I am very tempted to buy the Hanna Ca Checker.
Mg - I have only used the Salifert test for no other reason that I trust Salifert and never had a reason to look at other test systems.
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