I think that it is a great thing that your parents are willing to consider letting you ride a motorcycle if you can persuade them via a research paper. While you said early in your original post that you are having difficulty discussing it with them, it sounds like they are willing to discuss it if you put real thought into the issue.
I guess then that us giving you the highlights would be cheating, now wouldn’t it? I know, I know, I’m terrible.
My suggestion would be to take your time and do it right. Don’t rely on anecdotal evidence from the forum. Instead, look for published sources regarding the pros and cons. My suggestion would be to start with David Hough’s Proficient Motorcycling. This one is a very balanced look at the risks of riding a bike and how to manage those risks.
The Motorcycle Safety Foundation also has a book called Guide to Motorcycling Excellence. This is basically an expansion on the book that they give you for the BRC. It doesn’t have as much information about actively managing the risks of motorcycling, but it does talk about basic riding skills development.
I read the MSF book first and then Hough’s book, but for what you are doing Hough’s book will be the better resource.
Also, for the pro side, you might think about putting together a dollar cost analysis for the two vehicles. Insurance, maintenance, fuel, gear, etc.
Finally, use footnotes, endnotes, appendicies, list your references. Maybe put in an appendix with specific information about the 250 that you are interested in. Put it in APA or MLA style. Do whatever you can to make sure that they know that you are serious about it.
Best of luck!