Since 17.9ppm CaCO3= 1dKH, the ppm method is likely more precise. Is it the type where 1 drop = 10ppm? In the dKH types 1 drop = 1dKH (unless you double or quadruple the volume), so they are less precise than a ppm-based kit. But if you use the pH-kH-CO2 chart, it really doesn't matter which one you use - it is not that finely divided in kH.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.