I have been using API for 10 years. For the last 5 or 6 I've had Salifert test kits as well. And I have 2 systems (400g & 200g) both with lps and sps corals.
I test with API because I have found it so easy to use and just as accurate as the other brands. Here is what I do. I test with the API kit and if I get a wild number I didn't expect, I test it again to confirm. If I confirm the wild number with the 2nd API test, then I do the same test with the Salifert test kit. Way, way more often than not, the Salifert kit just confirms the API number and is so close that I have complete confidence in the API kits.
BTW, I tried the Hanna test kits and hated them. I found them to be a giant PITA to use.