Draining does a set amount plus a percentage, generally it's worse off than a pure percentage based brand on a heavy weapon (It's worse than flaming/frost for example on anything with a base damage over about 14) and it's generally worse than all the flat damage brands (disto, elec, pain) on a fast weapon. It's usually better than vorpal.
Venom weapons add a levels of poison per hit, the amount of damage per round that a creature takes from poison depends on how poisoned they already are.
If P is how many levels of poison they've got on them then:
Henzell wrote: [If 50% chance: dam = 1d(P+1)]. [If P>=4: dam += 1]
Is how much damage they'll take per round (And each round the poisonedness goes down too)
So let's say you hit them 10 times with your poison weapon, and they lose one level every round They'll take:
(1+11/4)+(1+10/4)+(1+9/4)+(1+8/4)+(1+7/4)+(1+6/4)+(1+5/4)+3/4+2/4
(or 22.25 damage on average, or 2.225 bonus damage per hit.)
The more times you hit something with a poison attack the more damage it will take on each of it's actions (So poison is more effective on a fast weapon, and more effective on slow creatures, and creatures with more hps, since you can stack more of it on them without it wearing off)
fsim does not account for the poison brand.
Also most insects are vulnerable to poison (Will take roughly triple damage from it) also it's nice to be able to walk away while the thing continues dying.