It's the AF one. The F100 does have a DoF preview button, and I've made a habit of using it when I'm not shooting wide-open.
The thing about the focus indicator is that I feel like it defeats the purpose...When I want to use manual focus, I want to use it because it is faster, or it lets me focus on something not on an AF point. Using my brain can sometimes be faster than selecting an AF point, even if I'm in MF.
That is why I love manual focus...It lets me focus in on any part of the scene faster if it isn't behind an AF point. I guess rangefinders somewhat defeat this purpose, but, eh. :\