If you had the choice between Particle Illusion and Trapcode's Particular plug-in for After Effects, which would you choose and why? For the purposes of discussion, let's say you already do the bulk of your motion graphics work in After Effects and you're looking for a flexible, powerful partlcle system. I will post the same question in the Trapcode forum, in an attempt to hear from both camps.
If you want a 3D particle system that works directly within AE, but that has very few available preset efffects then particular is for you.
If you want a 2D particle system not inside AE (but that works pretty well with it) that can *often* but not always fake 3D, works very quickly, and that has a whopping 1400 preset Effects, then particleIllusion is your tool of choice.
If you have the money for both, I'd suggest you buy both. Each has strengths and weaknesses. There's no right answer.
I've found that pIllusion and AE couild be made to work very well together, BTW. You just need to know how to do that. It;s all about workflow.