As this is the Advanced forum, I like to look a little deeper into the subject.....so bear with me....
A lot of questions and problems about testing kits, comes from lab practices. I think that should be mentioned first.
1. Rinse your glassware and syringes before and with the water, to reduce contamination.
2. Always use fresh kits, and make sure they have not been exposed to head.
3. Use the same syringe for adding the same reagent each time....don't use one for all.
4. Its really best to do the test twice.....even if you get the result you are after.
5. Have a "standard" if that's possible. For instance I bought a glass NIST certified thermometer for temperature verification.
6. Look at your tank before testing? Does everything look normal? This may corelate with a testing result.
7. Use a couple of brands for verification. (Got burned on an alk test more than once)
8. Top off your water first.
9. Get a refractometer.
10. Unfortunatly, you get what you pay for is what I have found with regards to this.
11. Make your choice somewhat based on what is available local, at least for a secondary in case of an emergency.
My personal preference: I found Elos to be the most straight forward and most reliable. I used them for all in the end, but used Salifert for the other "harder to find tests". I found LaMotte very professional and accurate for the dkh and would use them for precise measurments as it would go down to the .1 dkh. I used a Hanna Calorimeter for P04, but found the Elos to be comparable....
All kits have their faults, however most lie in Operator Error in my experience.....so look there and use the above to minimize the rest, which will help regardless of your choice.