A fast lens is handy but won't always produce good shots. Because the depth of field is so incredibly narrow if the auto-focus is even slightly off you can completely miss a shot. Then there's the fact that most (budget to mid-range) lenses aren't tremendously sharp when used wide open - especially at the corners.
Low noise at high ISO is far more helpful. You're still at the mercy of the camera's auto-focus but you'd be able to stop the lens down, increase the ISO without risk of noise, and have a greater margin for error.
I have a 35mm f/2 on my Canon XSi and it's gloriously sharp between f/4 and f/8, all the way to the corners. If the auto-focus is a little off, I've got a greater depth of field and can get usable images. Unless I'm in good light, auto-focus at f/2 is a lottery.
Give me low noise images at ISO 3200 and above on a camera which doesn't cost £2k and I'll be a happy camper.
