Folly Beach is the beach that we prefer. It is not run down at all, it is just much more laid back and isn't "yuppified", as IOP or SI is.
No matter where you are in the area in the summer months the traffic is not easy. But, even on the 4th of July it was easier to navigate FB then Mt. P traffic.
From what I understand there are more rentals in SI than FB. But the entire area is lovely and we can't wait to get there!