Some stuff I figured out when I first got AR (8 or so years ago?):
Powershield is max 8 frames from the point when you start shielding.
If you press down your shield too slow and lightshield the first frame, you only get 7 frames.
If you press it even slower and lightshield more than one frame, you lose the whole powershield.
If you start shielding before you can actually shield (before a move ends), those frames will count as lightshield frames (more than one and you can't powershield).
"Reflectorshield" works the same way, except it's 6 frames shorter (so either 1 or 2 frames).
Resetting your shield button to disable lightshield will always give you the full 8 and 2 frame windows (though it won't protect you from trying to shield when you can't).
Powershield has the same hitstun as a normal shield, but after the stun is over, you can do whatever you want. So if you let go of your shield before this, you are free to do any ground attack, start running, etc.
Powershield has about the same knockback as lightshield. If you want to stay in range for a shieldgrab, you're better off with a regular strong shield.
"Reflectorshield" has no stun or knockback whatsoever.
edit:
I was wrong about some things,
http://www.smashboards.com/showthread.php?p=13282543#post13282543