I used to live in Destin, and there is just about anything to do there, that you could do in a larger town. Putt-putt, water parks, great restaurants, nightlife, fishing, etc.
When we go back to visit, we always stay in Sandestin. It's actually one of the closest resort areas to 30-A, which is where you'll find Seaside, Watercolor and Rosemary Beach.
Watercolor looks nice too, but IMO, it has more of a resort look to it, whereas Seaside looks more like a retro community. Rosemary Beach has a more Spanish-inspired style, but I would liken it more to Seaside in appeal, than Watercolor. Destin is in a whole category of its own.
You can also find some nice rentals in Grayton Beach. In the order heading into Panama City from Destin, you'd find Destin, Grayton Beach, Watercolor, Seaside and then finally Rosemary Beach. Each place offers something different, but if you're looking for white sand and that blue water, it's all going to look the same.
To put it simply: if I were looking for an active vacation, with plenty of beachtime, I'd stay in Destin, and take day trips. If I were looking for nothing but beach time, with a little bit of lazing around, I'd stay in Seaside. It's a very cute town, but there is not a lot to do there, and it's going to cost more to stay there.
JMO - YMMV