Different strokes for different projects. Ones that need 3d particles and motion thru 3d space, Particular is great. For projects that dont, and benefit from the wide variety of presets (AE plug soon?), PI is my choice.
Just depends on the effect your trying to achieve. I personally will choose PI more often when it resides in AE. Timing and expressions and blend modes are often crucial to speedy success and these will only happen when its a plug.