The P2 slide switch can't really turn on in your pocket unless it snags on something, or if the LED leads get smooshed inwards. And the P2 intrinsically has a momentary mode--just squeeze it. The Freedom is more water resistant, and its dimming feature is handy, as are its other features if you can remember how to use them. What can I say, I have a Freedom and have never been that crazy about it, so I carry a P2. YMMV.
The Arc AAA is a real nice light and I love mine, but for typical occasional keychain usage, it's never going to beat cheapo coin cell lights in terms of cost effectiveness. A 50 cent coin cell light can easily last for years.
Note: the Photon series lights with red leds use a fancy HP/Agilent led that's unusually bright as such things go (much brighter than the cheap Countycomm red-led lights). Good on them for using such a high class part, but if you want a dim red light for briefly checking a readout, it might not be what you want.