I have both of these AF test kits , and they both seem reasonably accurate and easy to use. I'm using them on a newish tank that had some LPS and SPS frags added a few weeks ago.
Both kits were testing in line with what I would expect from the salt mix, and both picked up fairly subtle drops in CA and MG after the frags were added. Not exactly a peer reviewed double blind study lol, but I can say "so far, so good" with some confidence...