But I don't think they are necessarily opposing each other. There's something to be said for just getting the first draft down regardless. I've got a great book by Scott Meredith, who was a long time agent in the US, and he says you should get into the habit of writing the best you can first time.
I think it's a valid point. There's speed writing and there's writing fast. Speed writing I would regard as just getting something down, anything, so long as you keep writing. Writing fast I would categorise as doing the best you can as fast as you can.
Nanowrimo, where everyone writes like mad with the objective of doing 50,000 words in November, seems to prove out the belief that writing intensively is the best way to keep going. With a deadline looming, my buddies and I talk of doing a "sprintathon" to get finished. Which effectively means writing like mad for about a week or so.
I've discovered that I write best when I write fast, so I now try and get up to four writing days a week and expect to keep up 5000 words a day, which takes about 4 to 5 hours with a couple of breaks. Why this works for me is that I don't think. I just write. The composition goes straight from my "inner" brain, if you like, to my fingers. I'm a touch typist and I guess that helps too.
In the non-writing time, I plan the next bit. I know when it's going well because I start plotting - in the bath, while cooking, etc. New ideas jump into my head. And when ideas for the next book start jumping around, I know I'm really moving.
Learning to write fast means you have to push through a series of barriers. These are the very things that get in the way of self-belief. You could call it cutting out the middle man.
Now I'm not suggesting everyone should write fast. You'll have a comfort zone that you like. I do suggest that once in a while you give it a try. You never know, it might just work for you too.