We've been to both Outer Banks and Edisto Island. I think it depends so much on what you like to do. Edisto Island is very quiet and laid back. Not very touristy at all. Great for nature lovers, golf lovers, and beach lovers. If you're staying at the Fairfield Ocean Ridge, the condos come with bikes, and you can ride everywhere. We don't golf, but the golf course looked really nice. There are some good restaurants but very little in the way of shopping or any other commercial activity. Edisto is just 45 minutes from Charleston, so if you want shopping, culture, history, and even more good restaurants, it's close at hand. Savannah is a couple of hours away and is well worth the trip.
There are a lot more activities available on the Outer Banks; it's also more crowded and more touristy.
We enjoyed both, but for different reasons.