We've been to the Atlantic beaches (Cocoa, Daytona, New Smyrna) as well as Clearwater Beach. Clearwater wins by MILES in my opinion. The beach is a powder white sand, clear warm water, great family beach, tons of stuff to do and places to eat at in immediate walking distance. From Orlando, its about a 2 hour drive and you're inclined to hit some traffic. St. Pete Beach is another good option there but probably about the same driving distance.
The Atlantic side will be closer with less traffic, but its just not as nice IMO. Rougher, cooler water (not cold this time of year though), darker sand beaches, less built up in the immediate area around the beach. You could certainly have a nice day for a few hours, but wouldn't be my first choice.