I researched some destinations last year for about the same time of year (went to St Lucia and it was great). If you were looking to go in the middle of the summer, you would look at the destination and its kiting, lodging, price qualities etc. But once the Fall hits, you have to look more at the weather.
Will you get wind? Some quick numbers I found... The % chance that any day in September will have kite-able wind (over 13 mph) are the following: Aruba 86%, Maui 84%, St. Lucia 65%, Margarita/Coche 59%, Barbados 49%, Antigua 48%, Cozumel 38%, Dominican Republic 32%, Puerto Rico 30%, Pattaya Thailand 21%, Oahu 17%, Belize 16%, Turks 16%, Phan Thiet/Mui Ne Vietnam 7%. These are some numbers that should play some part in your decision.
Will you get too much wind? That is hurricane season for many destinations of choice, so go here to see what your chances are of seeing a hurricane at various locations (this site covers just the Caribbean, but other sites exist for the rest of the world)…
http://stormcarib.com/climatology/And finally, reach out to Kingsley Thomas Wong, he gave me some great insight, as he has pretty much been everywhere.