My Gods. Neither.
I have a bit of a problem with either side of this argument, which could be because I mainly hear this from those on the extreme ends of each side, but still.
Yes, I find it sexy when I woman has nice breasts and exposes them.
No, I don't think it gives them any power they didn't have before.
No, I don't think that is makes them a sexual object.
The situation that they reveal their breasts in is important. If it's something that makes sense for the character, then awesome, if not, lame. There should always be a reason. A better reason than "Oh yeah fan service." If a woman shows her breast to try to seduce the audience into liking the movie, show, or what have you, then that's bad. If they do it to seduce a man onscreen, or something like that, great.