1 Mind = 3 Vit = 5 Healing magic. Sirona's gives 3 mnd, 3 vit, 10 healing. 3 + 1 + 2 = 6 mind. Curing amount is flat, you don't get more or less for stacking one of these stats or the other. So if you're talking cure amount only, Sirona's is equal to a 6 mnd ring.
It comes down to what you want to get out of it as a secondary use. 6 mnd ring will give you better mnd-based enfeebles since mind has a direct connection to potency on stuff like slow. Sirona's will make your cursna better but let's face it, trying to cursna doom is ALWAYS roll of the dice. I'd say 6 mind has more uses, but it really doesn't matter.
Edit: Obviously aquasoul wins. If you're a career Whm (or Rdm but for another reason) you should have the 7 mnd, if not who cares.