I've thought about it and thought about it, and I don't have an answer.
I think if I had to choose, I would probably save the individual and try to catch the serial killer later. Why? Because I figure if I saw the killer, I probably got a pretty good look at him. Later, I could go through the police files on the Internet and find his mug shot. If he (or she) is a serial killer, they have probably been arrested before for petty crimes. And their finger prints are probably on file too. Once I find the killers picture, I can find out the name and the address. Then after all of that, I can hopefully catch him or her. I would work very fast and hope I nap the killer before they have a chance to hurt anyone else.
In the meantime a life has been saved.
Also, I tend to react to what is in front of me. If I see a suffering person, my mind would be trained on helping that person and nothing else. I can't stand to see anyone suffer. The possibility of a serial killer killing someone at a later time would be very abstract to me. The person in front of me would be very real and my instincts would force me to help that person.