HTH 6-way has OTO testing, which is desirable for daily testing, but (as Dave notes) doesn't replace the FAS-DPD test. Also, the 6-way tests TOTAL hardness, not CALCIUM hardness. (K2006 tests calcium). And the 6-way only has enough reagent for 3 CYA tests, instead of 9 (I think) in the K2006.