I don't have anywhere near enough facts to tell you what to do, or what I would do in the same circumstances, but. Chances are if you've never built before you'll run into cost over runs, which will shrink the difference. You'll also be waiting to lock in a mortgage rate, which has just recently come back down from some recent highs.
Unless building from scratch is an experience you've always dreamed of having, you may want to consider the nearly finished home and at least be in a comparable neighborhood at present market value. The next question is how quickly can you sell the lot and how much will you lose going that way?
There are many issues to consider. Please come back and let us know what you decide when you do.