Haven't tested this specifically, but, every test I've ever done (that had statistically significant results) has shown "Less fields in a form is always better".
"Less fields in a form is always better" is not entirely accurate. I'd agree with "Less fields in a form increases the number of people that complete the form", but this is not necessarily "better".
It does not mean that people will actually read the newsletter or complete the checkout process or come back and spend more money, those are the real metrics to track and rarely have anything to do with how many people submit the form.
