The p1 is single speed but there's two versions of it. The aluminum one with dual 26" wheels, and the cromoly with 24/26 and a gyro.
The p2 is heavier and uglier (hehe), and it has gears.
The p3, probably a bit lighter, and it has better stuff on it. But still gears.

I think you could put together a pretty sweet bike with the Steelhead frame; I was toying with the idea myself. But $300 for a bmx seems more viable than $750 or so for a build with the steelhead frame and lower-end components.
Then again, I'm only 5'11, and a bmx bike wouldn't seem quite as small to me.