Vectra is right in all points. Of course you inflict more wounds with poison. This is the point about poison anyway, isn't it? But lets assume:
Poison, without reroll: 40 attacks, 20 hits (on 4+), 6.66 of which are poison so no roll to wound, 13.33 need to roll > 2.22 wound (T5) = 8.88 wounds...