I use API too. A couple of years ago my LFS carried Hagen liquid test kits. Their color charts were different from API and were easier to read. Not sure if Hagen stopped making them or if my LFS stopped carrying them. :-?
Since all require a color judgment, I go with the test strips for ease of use. Once established the tank will rarely have nitrite if filtered correctly so I don't stock a separate kit for that. Kits, whether liquid or strips do get old and need to be replaced. That further drives me to the Jungle 5-in-1 strips as I test often and it gives the full readings of nitrate which is the one I watch most and then if there is a mess up that gives me nitrite readings it will tip me off to them without any extra effort. For me it is a question of doing the test because it is easy but maybe the results a .2 off from true which gives me no heartache or maybe not doing the test because it is a pain and then the fish get funky before I get the proper true reading. My eyes are not reliable enough judging colors to worry about a small amount of fault in the test kit.
I have 2 kits the API and the Hagen(I'm going from Memory but I think my second set is a Hagen) The API "Nitrite" kit stats to register at .25ppm indicated by a shade of yellow that approaches green, the Hagen starts registering at .1PPM indicated by a very light blue (if its not clear then nitrites are present. (again going by memory) I will verify the numbers when I get home.
Hagen registers 0.1 ppm with a light pink/magenta colour API has varying shades to 0.25. But you can probably guess close enough in between to be comparible to the .1 ppm. Probably not much difference.