http://www.crys.org/everquest/spells...k=1&maxrank=65
Your disease line of DoTs will always be more mana efficient, but the poison DoTs are also very useful. The poison line dots have a short timer, 42 seconds, making them very useful for short fights (or fights that you intend on keeping short), where 1 ebolt, or 1 ebolt and a VotS are enough.
As you level higher the mana efficiency of your spells increases, and as does your mana return thru cannibalize due to the better regen spells, so the mana hit on a poison dot won't seem as bad. Also, keep in mind that the faster the mob dies the faster you can pull the next one :P