In my opinion, no matter what, Toxic will always be the better option. While poison does a constant amount of damage each turn depending on the opponent's HP, Toxic does the same thing, but every turn, the damage increases to dangerous levels. Besides, there will be Pokemon that are immune to poison, like the Steel-type and Poison-types themselves, so just spreading some gas over the opposing team may not affect some members of the opposing team at all. Besides, an opposing Pokemon being intoxicated will almost always result in a switch out if the opponent doesn't have a good strategy against it, leading to almost the entire team getting intoxicated soon enough.
tl;dr Toxic is the better choice, due to the increased damage.
Source: Knowledge
