How about Europe? The day time would still be long in Sept so you could have more time for sight seeing + photo taking!!
It will be the low season so the flight and the hotels won't be too expensive.
10 days or more would be the best...
Most flight to Europe would fly at night. You can still work on day 1!
But if you only stay in one city, a week would be pretty enough.
anyway, for 5-6 days, Asia would be better.
What is your purpose?! Sun and beach? Shopping? Photo taking? Sight seeing?