I would corroborate what Greg just said, FWIW.
In my opinion, for whatevr it's worth,
With the same finish Activator AND Acitvator+ is stronger than PK18 and will handle more oil than PK18. In their respective out-of-the-box finishes, these 2 balls for for different, if not complimentary purposes. They can be mixed and matched by changing surfaces until doomsday, as both coverstock are almost infinitely adjustable.
With some polish and some drillings, the Nemesis can appear more flippy than the AI; I think that's due to the their cores. The AI can appear somewhat flippy, but there's fewer circumstances where that might happen.
That said, the Activator+ is an absolutely amazing a coverstock to my mind and is as much a break-through in bowling technology, as the whole idea of particles has been since their introduction in the late 1990s.
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Bowling: Just like hand grenades and horse shoes, you only have to get close.
Life: Deal with what is.