I've only used one - API (liquid). I have heard that liquid tests are more accurate from a number of sources. Some of the colors are a little off or hard to read, but you do what you can.
Planted 10 gallon tank; 2 x 10W CFL; Fluval U2; MGOCPM/PETCO black sand cap
The Fraternity of Dirt #152
A Silent Cycled Tank