Ok, I'll go first. You won't have a shortage of answers from the Peanut Gallery; but don't hesitate to also look at old threads; lots of great input of similar questions.
Where? Depends. Of course.
--Keys--Shallower, if your group is less experienced. Plenty of ops to choose from. Not so much night life. Some deeper wrecks available.
--Ft. Lauderdale--drift diving from boats; wrecks and reefs available, both shallow and deep. Shore diving is plentiful. Night life/outside activities of any sort.
Holiday weekends are ALWAYS nutty-busy, so be prepared to have very full boats.
Early September will be warmest, barring strange weather patterns.....water temps in mid-80's range. Viz? Well, assuming the recent spate of hurricanes is over, anything from 30-ft or so to over 100 ft.
Excepting hurricanes, September will still be stormy season--expect good mornings, getting exceedingly hot (uper/mid 90's) til @ 2-3, then intense thunderstorms for the mid-afternoon. I generally stick with AM dives for this reason.
Enjoy the trip; let us know when you're in town; we almost always have a Conch-Dive going somewhere! :dazzler1: