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, 2x - T5HO, Eheim 2213 and 2013, red tiger lotus, vals, java fern and moss, crypts, swords, mbuna tank: Yellow labs, Ps. demasoni, clown plecos, SAE's.
10g mini-reef. 16lbs LR, green mushroom, star polyps, wagon wheel polyps, purple mushroom, candycane, hammer, other zoas, CUC.