I'm going to have to go with free will. I know that I chose what I am going to do or not do.
As far as Physics go, well, speaking on an atomic level, you can't predict how everything will happen. So there are things that happen "randomly". A simple experiment; a tennis ball, a room with no ventilation and a release mechanism. Drop the ball 100 times, it will not go in the same direction, upon bouncing, every time. Things do just happen sometimes.
As far as humans and the way that we interact with one another, well, if we are going to say we don't have free will, how can there be any laws to punish those who do wrong? Is this going to be some kind of "scapegoat" eventually that everyone falls back on? This kind of thinking furthers my belief that mankind has become so weak, timid and frail, that they simply refuse to take responsibility for their actions.