I have to agree with pwr4all3.
P21S wipes on and buffs off without hazing, while #16 is liquified then applied and allowed to haze before buffing. P21S doesnt stain trim while #16 does. P21S is a beauty wax that imparts clarity and surface reflection at the expense of durability due to a beeswax/carnauba combination, while #16 is a heavy carnauba with less shine and provides longer durability at a moderate sacrifice of appearance. P21S is very easy to use and is very consistantly predictable, while #16 can be very difficult to use if allowed to cure slightly, contact trim or applied in sun or high humidity.
I dont want to be accused of disagreement, so you see, they are the same.
NHBFAN, your incessant Meguiars bashing has to stop! Please join us in telling the world the real truth about wax. I recommend you practice 1 hour of Meguiars mantra every evening for one week. Ive never waxed next to Mike like Randy has, but I'll bet its a spiritual moment.