I use the API kit for basic Ammonia, Nitrite and Nitrate as well as pH. It's one of those things that you really don't need to monitor beyond a range that's good enough or not present at all in case of ammonia or nitrite. API kits use glass beakers which are better in my opinion than plastic beakers which can stain and make it difficult to read the correct colors. API's plastic color chart is also a nice feature over paper ones in many other kits.
For other trace minerals and metals, I like Salifert or Seachem kits. Colors seem to be more distinguishable in smaller increments in general with these kits. Though with the plastic beakers, you must bleach them one in a while to keep it not tinted or stained.
For copper medications (for QT fish), I would agree with everyone in the industry to use strictly Salifert Copper kit as it will measure all forms of copper and at smallest increments and is easy to use.
Just my opinion here.
