Inspiration vs. Writing On The Spot

A few weeks ago, Tabitha Olson posted on her blog about writing on the spot or waiting for inspiration. Its really a great post. (http://tabwriter.blogspot.com/)
To me, its much easier to write when you know what you're writing about. If you have a certain story in mind, it flows out onto the page more fluently.
However, there have been times where I sat down to write something, and a story just flew out of nowhere. So, sometimes writing can just come on the spot; you just have to be ready for it.
Whether it's inspiration or writing on the spot, I like both. What's your opinion?