I'm gonna talk from a competitive standpoint as ingame it really doesn't make much of a difference.
Spore is better and by a large margin. It's better because of Amoongus' role in a team. Amoongus doesn't deal damage, it supports team members. And to do this, Spore is significantly superior.
First of all, lets acknowledge exclusivity. Spore is rare, and is usually on pretty mediocre Pokemon. Amoongus being a fairly good Pokemon with Spore is huge. Lots of Pokemon can learn Toxic.
Honestly, if you want a Grass/Poison type to learn Toxic, use Venusaur. It's bulkier and hits harder. But it doesn't have Spore.
But why is Spore so good? Well being able to put something to sleep is extremely disruptive. If a Pokemon is poisoned they get worn down sure, but they can still attack and be harmful for a significant period of time afterwards. If a Pokemon is put to sleep, they're straight up immobilized for a number of turns. No setting up the next Pokemon, no sacking the poisoned Pokemon to gain the next turn advantage, no quickly knocking Amoongus out anyway, no switching out and then back in later with still pretty decent health left. Straight up useless. Unless the Pokemon as Sleep Talk, once they're asleep they're as good as fodder.
The most you can do to play around Spore is to have your least important team member to take the sleep so it's not a big as a detriment, but that still essentially makes it 6v5 until the Pokemon wakes up. That's a huge advantage to have. Of course having a grass type definitely helps deal with Spore, but having a Poison or Steel type helps with Toxic too so it's a bit of a moot point if I'm honest.
Spore grants free switches to your teammates, allowing them safe entry and even time to set up their own moves. When a Pokemon gets spored, 9/10 the asleep Pokemon will be switched out to avoid giving free turns to the other party, but that sleeping Pokemon will stay asleep while in it's back, so it's rendered useless for anything except taking hits.
Spore works even better on Amoongus thanks to it's great Regenerator ability which allows it to come in and out several times more than otherwise, constantly threatening the opposing team with Spore throughout a match.
Lets wrap this up shall we?
- Lots of Pokemon get Toxic, few gets Spore
- Spore offers free turns to set up your strategy
- Spore disrupts your opponent's strategy
- Spore renders your opponent's mon effectively useless
- Amoongus can use it a lot through a game thanks to it's ability, keeping constant pressure
So yeah, use Venusaur if you want a bulky grass mon that knows Toxic.