The advantages are numerous, especially if you don't mind lane splitting... which has been demonstrated to be quite safe in a California study... it's only the doofi who don't do it right that get hurt. Parking is usually easier and cheaper.
Primary disadvantage is rain... even with good rain gear and boots it is still easy to get wet. And a key issue is visibility... for you to see and others to see you... I have generally tried to avoid rain, although I did ride almost the whole way to KL in a heavy storm.
Be sure to get the book, "Proficient Motorcycling" (
http://www.amazon.com/Proficient-Motorc ... 1889540536). One read and you will realize how many things you have not been looking out for and it will greatly increase your chances of not being part of an accident scene.
I'd definitely stay away from the Harley. Besides maintenance costs they are simply not as reliable. You'd do best with a Japanese brand bike in terms of reliability and ease and low cost of maintenance. Liquid or oil cooled is best.
I'm not a scooter person because I don't like automatic transmissions or the smaller wheels. I'd probably opt for something between 400 and 800 cc's that is small enough to filter traffic easily.
I used to ride one of these... could still lane split but there were limits.