Preplanning vs Improv

Lately I've been debating at least in my own head about whether it's better to go with your first instinct and the power of the original idea or to sit down and calculate what you want you really want out of an image. With spontaneity you might miss out on a really great fleshed out idea. You could be spending hours on hours on an idea that just isn't as good as if you would have really worked out every detail before starting on the final version. However you can really loose a lot of inspiration, looseness, fun and carefree feeling to an image if you over work it. Alas, I do not have an answer yet myself. I'd love to know what your thoughts are. As of right now I have a couple projects that I'm working on that are pretty planned out and a bit more fully thought through, and a few that are just, here's an idea, here's some paper, let's go. I suppose it's finding some sort of happy medium.