Personally, I would attribute at least part of the difference in google hit rate to the popularity difference. The salifert kits are more expensive and not nearly as popular as the API ones. (I do have a salifert nitrate test and an API freshwater master kit along with a few other API kits).
Rather than buy another kit, make yourself some known-concentration solutions and test those..
If you get a second kit and they disagree, you won't know which is right (or if both are wrong). If they agree, you won't know if they're both right or both wrong.
I ended up calibrating my ammonia test when I did ammonia-based fishless cycling. I used a dosage calculator to know exactly how much ammonia to add to reach a 4PPM. I let it circlulate in the new tank and tested to confirm. I later repeated this with 1ppm.
Rather than an un-cycled tank, you could probably do the same in a 5 gallon bucket. At that point you could even use sudsy or lemon scent ammonia, as long as you know the percentage of ammomium hydroxide in it. It won't be fish-safe, but we're not planning on tossing fish into that bucket.
My API ammonia test kit was dead-on. My API nitrate kit reads wildly too high (4-7x too high)
Last edited by mattinmd; 11-04-2014 at 05:50 PM.
Reason: added API test to my list of tests owned.