Both are pretty dope, but I really like Stone Mountain because after you're done hiking the mountain, you can hike around the mountain. Until recently, I didn't realize that they had wooded trails and lakes surrounding it! You can make a pretty good day of it there. Plus, aside from the $8 or whatever for parking, it's totally free. Bring a picnic and you're set.
I shouldn't discount the High either, especially right now. It's a huge deal to have the Louvre tour parts of their collection to right here in Atlanta, and certainly worth taking advantage of. Plus, it's real easy to get to via Marta. Up the steps, and voila.