Two different things entirely, in two different counties.
Destin is an actual incorporated city in Florida, in Okaloosa County. It extends from the East Pass into Choctawhatchee Bay on the west to the Okaloosa/Walton County line on the east. As its own city, it contains not only condos, but also houses, schools, retail, marinas and docks, an airport, etc. It is pretty well built-up and Hwy 98 gets pretty congested.
Sandestin (one word) is an inclusive Golf and Beach resort about 3 miles east of the City of Destin, in Walton County, Florida. The resort features stand-alone homes, villas, hotels, condos, townhomes for residential. It has four golf courses, tennis, swimming pools, other recreation activities.