From what I read about bipolar vs borderline is the bipolar is a medical brain chemistry issue and is more unpredictable and uncontrollable swing and borderline mood swings are actually not uncontrollable but are part of a personality disorder, where the sufferer is choosing to behave in a manipulative way, or is reacting to emotional stimulus rather than biological.
disclaimer - I'm not in the mental health profession, just read a couple books on borderline personality