the P1D is a twistie : you twist the head on and off, much like your standard minimag and solitaire. Twisting the head the change modes can be annoying to some, and although possible to operate one handed, its not easy
P2D is a clickie, meaning its the click switch, i am assuming its a reverse, meaning that when you click it in, nothing will happen, only when you release, the light will come on. This is alot easier to use then a twistie, and can be operated 1 handed. The disadvantage is that clickies have more tendency to fail then twisties. Twisties are pretty foolproof.
the only other differnce is that that P2d is slightly bigger, which is why i went with my choice...
Crenshaw