In my opinion the Hanna tester is more accurate for a few simple reasons (remember, this is just my opinion). First, you are comparing a digital readout to the ability of your eyes to match colors that are typically quite close. Second, most test kits have a gap in their result numbers. It'll let you know if your po4, for example, is 0.01, 0.03, 0.08, 0.12. But what if your po4 is actually 0.05? You're expected to guess where it lies, once again, based on your eyes judging a color and estimating the number. You are not given an exact number. A good example of issues with these test kits would include the Red Sea line or any other type of titration test kit. When performing these tests you're forced to distinguish and ask yourself "Is the color changed to red now? It's not the original color...it looks red, maybe a small touch of pink? Do I take my reading now? Let me add a few drops and see. Ok, look similar, but far from the original test color. Is this my result?"
You can see the issue. There is a lot of guesswork involved with non-digital test kits. That alone will more than make up for the small margin of error allowed with the digital kits. And Last of all, once again just my opinion, I will trust a near lab grade digital instrument over a liquid/powder mix with water test kit anyday. I think the major key to its accuracy is hard to argue...it's easier to read, it's digital. That's enough margin of error for me.
