Mike,
If you buy the existing site, you will be the proud owner of a 35 year old flood prevention lake with a good fishery.
On the other hand, if you buy the other site, Meadowlark is absolutely correct. You run the risks associated with building a new lake, then stocking and managing it. Those risks are, designing the dam and the lake, working with contractors, soil issues, watershed management and stocking the fish.
Personally, I would buy the site to build the lake, if your temperament is such that you can work through the process of creating a new lake. It's quite fulfilling, knowing you conquered it, and it's your lake, from the get-go. Plus, when you build the lake, it's a fishing lake, not necessarily a silted-in flood prevention lake with a shorter life.
