The poison is stronger so it lasts longer.

Virulent may not seem like the best choice but I find it infinitely more powerful than Vengeful (I also think the poison is stronger but that is more in Zeddy's area of expertise). For each target a Vengeful quill hits the second quill only goes to one other person, and since the tight spread of the quill you will most likely only poison two people (one with main quills, one with the secondaries).

The thing about Virulent is it has a decent poison effect and can poison troves of people easily (if you know how). It has a high chance to explode into a mist that poisons anything inside it, making the use of them and vortex so deadly. The mist may seem small and short but the amount of them that you get makes up for that (btw zeddy we need to talk about the size, more testing makes it seem bigger than the mist really shows).

I digress though, it comes down to how you would rather play it. You can have a guaranteed second hit but on fewer targets, or you can have a chance at getting more targets, it all comes down to how you play. There is one thing Tweeter and I both agree on and that is Chaotic Haze is the way to go (even after they fix Haunted I am not sure how it will fair, getting a kill with a haze very rarely happens unless you aim for it).

Well I found myself using the Vengeful when it came time to choose. It's also very useful. But I gotta ask: Is virulent really stronger poison? What's the size of the explosions? Do the explosions do damage?

EDIT: I did NOT choose Virulent! >.>

@Xtweeterx

Judging from this thread Zeddy has found Virulent to have a stronger poison. Here is what the explosions look like (they do not do damage). They look tiny but it seems they are really one square bigger than it shows with further testing.

Now again Vengeful is a very solid choice (I think these two ultimates are the most evenly matched), what is loses in poison strength it makes up for in guaranteed second hits.