A while back I did a kitchen table test between API and Salifert.
With the NO3 test, Salifert was clearly more accurate. The API was pretty far off, surprisingly far off to me. Could it be related to "shaking"? Yeah, maybe but I have always really shaken it like mad. My guess is once it gets off, it will stay off. The Salifert is very consistent.
The API PO4 test was reasonably accurate. The Salifert PO4 only goes up to 3 ppm, which is pretty useless in my tank. At 3ppm my tank would be starving, so would never really be an option for me.
Here's a link to my entirely unscientific tests.......................
So for me, I use API for PO4 and Salifert for NO3. The absolute value is not that important, but relative values are. Basically looking to spot trends or anything wonky.
But more important than either are your actual dosing and the reaction of plants. I don't make decisions based on what I "want" the numbers to be, rather numbers that bring out the best in my mix of plants in my unique ecosystem.
For me, I try to keep NO3 = 30+ and PO4 = 5+. Any lower and hungry plants like Pantanal and other Ludwigia start to rebel. But as usual, your mileage may vary. You need to find out what levels bring out the best in your particular set up.