It depends where you have been. I would choose Paris if you are going in early fall and if you have never been there. It is a magical city and Marriott has some really great hotels there, such as the Marriott Champs Elysee.
I would not go anywhere in the Caribbean during September since that is the heart of their hurricane season. If you want a beach, Hawaii would be ok if you go later in the fall, though it's still not their best time of year.
For the time period you stated, Europe (Paris, Italy, Greece) has the best weather and also the least crowds, which means lower cost for everything.
What a nice dilema to be in!