We did not camp at Myrtle Beach State Park but we spent a day there and I walked through the entire campground. It was lovely. I would not hesitate to stay there or recommend it. We stayed at the Myrtle Beach KOA and were very pleased. Nice pool, fun stuff for kids, and fairly spacious pull thrus. It is two blocks from the beach but feels a world away.
Both of those places are awesome as far as I am concerned. Here are blog posts that I wrote on each of these locations with pictures etc.. Of course, the Outer Banks is awesome too!
http://livelylittlecampers.com/2014/04/sleepy-cypress-trees-and-the-cool-silvery-ocean-myrtle-beach-state-park/http://livelylittlecampers.com/2014/04/how-we-roll-when-a-campground-rocks-myrtle-beach-koa/