I have PI3 for windows and its is spectacular. The diversity and organic realism stunned me at first, and continues to do so. The plug in will be well worth the wait and will be released when Alan feels it is ready.
It is my understanding from previous posts here in the forum that there will be advantages to using both apps, and that there will be a competitive upgrade price for those who laready own pIllusion.
Having spoken to other plug-in makers, AE's architecture limits plug-ins in certain ways when compared to their original host apps - things like Open GL tend to get hit a bit hard becasue both AE and 3D plug-in (or open GL plugin as pIllusion will be) can run slower. Also, AE has limitations on how many parameters/properties something can have, an issue that pIllusion doesn't have. Just take alook at super emitters and see how many properties are there.
On the other hand, the advantage of workin in AE is pretty big, so there are times where there'll be no substitute for that.
All that said, Since the plug-in isn't out and there's no set release date, you are better off going for it. pIllusion is one of my top 2 programs (AE being the other) and it gets the job done fast - The 2 programs (AE and pI) work well together.