I was thinking that I heard that it had a 4 pin connector, so it changes speeds depending on the temperature of the processor. Let me look it up.
EDIT: Couldn't find if it has a 4 pin or not. I guess the main thing is if you have a case w/ a window, then buy the AT, if you have a window, then get the LED. The AT would have a tendency to be slightly quieter because of the black plastic instead of opaque, but you probably couldn't tell the difference.