Well here's the thing. Wraiths don't kill (usually), Wraiths are implied to be weaker than witches and so there are more of them, which is why you need multiple grief cubes in order to fully cleanse your gem, each wraith dropping a decent handful.
If anything I'd say maybe for every 10 wraiths = 1 witch but don't quote me on that. Remember that the Wraiths fight in packs so they have a pack mentality, vs the witches who cry in secluded areas and affect their surrounding Areas.
Wraiths are only dangerous if there are a large cluster, 1 normal average wraith on it's own can be easily defeated by a magical girl vs fighting a witch where it's all dependant on the girl's skill, anyone can die at any time.