I haven't seen any articles where each test kit was measured against a known reference sample, which would be great. There are numerous threads where members state their opinions, sometimes with conviction. For me, I found API to be difficult to use with my limited color vision. I tried most of the Salifert kits and found them to either be inaccurate (new mix with Instant Ocean measuring 540 for Ca for example) or else inconsistent, meaning that subsequent tests on the same water gave me different results. Some swear by them, and some swear at them...
I switched to Elos kits and although they're a little pricey, they seem to be both accurate and consistent. Some complain that the Ca kit from Elos only measures to the nearest 25ppm, but that resolution is fine for me since I'm basically looking for a reading above 425 and below 500. In terms of figuring out the effects of daily dosing, the consistency piece is the most important for me.