I've never been to South Beach, only Panama City. I hear South Beach is maybe the best but I can't confirm it. I love Panama City and I keep going back there. Sure, hit both if you have that kind of money to spend, no one else does.
In Panama City, I try to stay at a condo right on the beach, preferably Edgewater, its nicer than the other ones I've stayed at.

Main clubs to hit are Club LaVela and Spinnackers, although the waitresses at Spinnackers are total bitches, I prefer LaVela.

If you do spend a week at both, maybe hit South Beach forst, just cause you get an extra week of warming up for Panama City since its 7-8 hours north and cooler, you'll want all the extra warmth you can get. When I was there in '08, it was 50-60's most of the time. If we woulda went 1-2 weeks later, it woulda been much warmer than early March.