Definitely 6.0. Slightly acidic helps inhibit (not prevent) microbial growth, also not acidic/basic enough to burn new roots (as does occur at pH above 7.5 and starting at below 5.5 for some plants). It also is not acid/base enough to seriously degrade any chlorine product you might add. Personal anecdotal evidence suggests 8.0pH and above actually entirely stunts rooting. I had two bottom stem cuttings rooting for two weeks to nothing, three days of pH balanced between 6.0 and 7.0, nubs showing. New cuttings (just in case it took two weeks and three days to root) root pre-nubs in two days. Now for an accurate scientific study I would have to make sure my light didn't change as mine did. I realized about a week in that my light was way too strong and way more red spectrum than blue. As this can have significant impact on speed of shooting, proliferation of roots, success rate, and health of clones, it is mentionable.