I've not stayed at either yet (though I have a stay at the Sandestin property quickly coming up in April). But I've traveled to both areas frequently.

So to answer your 4th question on flying in. For Sandestin, you have 3 airports fairly close, but all are small "regional" airports (Pensacola, Ft. Walton, and Panama City). I'm not sure if any of the airlines actually fly into all 3 (i.e. AA doesn't fly to Panama City, Southwest doesn't fly to Ft. Walton, etc.) So being small there aren't too many flights a day and unless you are starting from an airline hub location (in the Southeast) you will probably have to connect to get there. But the good side on being smaller is car rentals are closer/quicker (and may be cheaper too). Crowds will normally be smaller, etc.

For the Diplomat, you have 2 airports that are larger (Ft. Lauderdale and Miami) with Miami being a pretty large and sometimes very crowded airport with a good chance of flying there with more options (i.e. maybe more award availability too.)

Personally, I've had no problems finding award availability for either locations though in the Florida panhandle, if you are trying to go in the summer months, you may have more trouble doing so.

The question is when are you planning on going? South Florida in the winter will most likely have better weather with the panhandle getting downright cold at times (and even for spring break can be a bit too chilly to swim as much.) But I like the panhandle beaches better. The sand is a more powdery white and wide. Also, what do you like to do while there?

Note that there isn't as much right there in Hollywood, FL though it doesn't take too much time to get up to Ft. Lauderdale where there are more options/restaurants/shopping/night life. Also, Sandestin is just east of the main part of Destin, but there are restaurants and shopping close by.