Ruin 1 or Ruin 3?

[KalinOrthos]

So I'm leveling my SMN and having a lot of fun with it, but...I'm a little confused as to when I should be using Ruin 1 or Ruin 3. Given the excessive mana cost of R3, I clearly shouldn't be spamming it, but R1 does such little damage. Should I treat the Ruins like Jolt and Impact, where I alternate every other cast, or is there a specific rotation I should be following?

Also, wtb Sustain please, SE

EDIT: I should have noted it in this post, but I do mean outside of DWT. In Trance, Ruin 3 is the only thing being used.