The K-2006 is the most recommended over the K-2006C because of the price difference. Most people are already somewhat hesitant to spend that kind of money on a good test kit even though it will save them lots more than that in the long run. However, the K-2006C has greater quantities of the reagents, so you won't need refills as soon if you don't mind paying an extra $40. (it does have more than twice as much of the reagents: 2 oz. vs. 0.75 oz. for most things.)