Whatever you decide to write your statement on, remember this: Make it unique. If, after writing it, you can see the same thing being written by someone else, rip it up. The PS is your opportunity to make yourself stand out, not blend in. Adcoms want to see from your PS that you are a person who is a good fit for the medical profession. Note that there are ways to do this besides directly addressing the question: "Why do I want to be a doctor?" A lot of secondaries ask you this question, anyways, so why waste your personal statement answering it? Avoid the cliche taglines and stories -- the adcoms have seen them all before. Try your best to present the person that is you. Good luck.