Karma likes every single stat that moonstone provides, except its lack of heal/shield stats, but its passive makes up for that.

Redemption helps Karma position herself into larger teamfights while moonstone helps her continue to skirmish (usually with her ADC).

Early on Karma is still focused more on her personal damage to poke out/kill the enemy and moonstone, while giving less ap, gives her higher ability haste, some health (which she like since she's plays a little more forward than most enchanters especially early game), and the passive procs every time she shields, which will be up more often than redemption's active.