What Type of Writer are You - a "Plotter" or a "Sit at the Keyboard and let Inspiration Hit Me" type?

I envy J.K.Rowling and her ability to plot not just one book, but seven, and do it all in advance, with clues to the denouement laid out carefully in all the books.

My problem is that when I lay out a plot like this, and then try to write the book, the writing comes out stiff and wooden. I work best when I sit at the keyboard with just the idea for the first chapter, and as the characters flesh out, inspiration takes over and I start to day-dream about the characters and then simply write what I've dreamt. Everything that happens is a delicious surprise and discovery.

I also re-write as I go, endlessly polishing what I've written, and frequently going back to change something or foreshadow events, as the direction of the book changes.

Is this an inefficient way to write? I don't know, but it should give you an idea of the trouble I'm having with the synopsis for my competition submission. For how can I write a proper synopsis when my book is unfinished and there might be a development that I don't know anything about?

The re-writing too changes the book. I was suddenly struck with the realisation that my opening didn't have enough conflict in it. So I had to go back and change the first three chapters. Good job I hadn't already submitted the first chapter to the competition!

If the worst comes to the worst, I'll just skip the competition and submit the novel in the normal way. Better to submit something that is good, than to hurry and submit something not fully thought through. The value of the competition is that it is making me write and think my novel through, rather than the prize (though the prize would be nice).