pH is pH, no matter what test you use, it should be the same. The different kits & range have differing accuracies, but pH is a physical property of the water that you are measuring.
Are you reading about planted tanks with CO2 injection? This is a totally different discussion as to what water parameters is best suited to a planted tank. <And they should be talking about CO2 level, which is a function of both KH & pH (plus other things), simply mentioning a pH value doesn't mean much ....>
Assuming that you are NOT injecting CO2, there are 2 competing factors in deciding what is ideal water for a planted tank:
1. You would want a slightly acidic pH (ie high 6's to 7.0) so the plants can get at nutrients (esp. iron) easier.
2. You want a reasonable KH (at least 2-4) for pH stability for the sake of the fish.
However, in the typical carbonated buffered water, the KH is linked to the pH. If your KH is 4, your pH should be 7.5. The 6.9 pH with KH of 4 requires artificial CO2 injection to 15 ppm. <Those numbers are actually really good for plants, but requires a high tech setup.>
For most people, you are best to stick with the water you have. There is no point chasing some mythical "ideal" pH or KH. <If you are injecting CO2, you should aim for a CO2 level of 15ppm. This will cause a drop of the pH, the actual final pH depends on what your starting pH & KH is. So if a site say pH of x is good for CO2 injection, without taking anything else into account, that would be wrong.>
