I would have kept Isla Nublar a theme park because I just have loved the concept of a dinosaur theme park ever since I saw the movie as a little kid. Another reason I would have kept JP a theme park is that people all around the world would flock to the island to see the magnificient creatures.
Now let me put this into a scenario. 15 years from now, cloning dinosaurs is succesful and all the dinosaurs would be put into a Nature Preserve. Think of how people would act when they heard this. "Dinosaurs! Back to life! I need to see them!" Well with a nature preserve you can't see them, thus people would continously have the want to see these animals. As the want and demand rises to see the animals and the company who created the preserve will not allow visitors. After so much want and demand to see the dinos, people start illegally getting to the preserve and interrupting the dinosaurs lives and tresspassing security. Once this is heard of, the genetics company would be accused of poor security and if people could get in, dinos could get out! If the illegal visitors were to be killed, this would raise more safety questions. Towards the end of the havoc, the government orders all dinosaurs to be eradicated from Earth, thus destroying the concept of the preserve and dinosaurs themself being on Earth again. Now see what I am saying? Maybe a nature preserve may not be a good idea and a park may be the better.
I am not saying a park would be perfectly safe either( as seen in the movie :P) but it might keep a genetics company up and running and people happy and satisfied from seeing the dinos.