I always opt for blue line prior to a positive rotation.
Let me play the Devil's Advocate here. What is the difference between normal rotation and "positive" rotation?" Forcing the aircraft to stay on the runway until 20 knots or more between manufacturers recommended rotation speed and Blue Line speed is getting into test pilots territory.
Be careful if using your own personal technique particularly if something should go wrong. Any Regulatory investigation would have you by the balls if it was determined you disregarded the manufacturers POH operational advice. The legal eagles would also take you to the cleaners.
You don't hear of airline pilots flying their 777's or A330's throwing away the book and using their own speeds. With all due respect you would be wiser to stick with the POH figures. Chances are you could increase the chances of blowing a tyre due using excessive speed on the runway. Trust the flight manual numbers. The investigators will.