We've camped at both, and they're fairly similar -- vast campground, tight spaces, lots of golf carts zipping around at all hours. Here's the upshot from my troops (both boys):
Ocean Lakes:
Better indoor pool, decent kiddie spray area
Better arcade
Pirateland:
Better fishing holes
Better outdoor pool area (lazy river, kiddie spray area, and big regular pool)
More trees (the few they have)
Other than that, I'd say they're close to the same. Good beach access.
That said, our favorite place to camp at Myrtle Beach is the State Park, without a doubt. I love the maritime forest, the huge pier, the hiking trails (there are a couple geocaches there if you're into that). It's not as glitzy as the others, but we adore it. (Much cheaper, too!) You can also have a campfire, which I don't believe you can at the other two unless you have a fire ring. I could be wrong on that point, though.
Have fun whatever you choose!