Back to Cookie Experiments
Brown sugar contains more moisture and so tends to produce a thicker, chewier cookie. Granulated sugar usually produces a crunchier cookie, but results do vary.
For example, here is the exact same cookie (the chocolate cookies of Stephanie Jaworski at Joy of Baking – a sinfully scrumptious cookie).
The original recipe called for 1/2 cup brown sugar and 1/4 cup white sugar. Instead, in the first batch, I used only brown sugar, and the second, only white.
The batches were made at the same time, and I treated them the same way* – mixing them the same way and cooking them on the same sheet. (I baked one sheet with the white-sugar-cookies facing the oven door, then the next sheet with the brown-sugar facing the door, to get a representative sample).
On the left, the granulated sugar cookie. On the right, the brown sugar cookie. These cookies began life in the oven as equal volumes of dough. The granulated sugar allowed the cookie to spread more, producing a larger, flatter cookie. The brown sugar produced a plumper, puffier cookie.
This recipe is for a quite moist cookie, so the granulated sugar version is only a little crunchy, but it is definitely crunchier than the brown sugar version, which is nearly brownie-like.
Neither is better or worse – pick the sugar that will produce the kind of cookie you're after: crunchy or chewy, flat or puffy.
* Experimenter's Note: At one point, Moosewood Book of Desserts fell off shelf above counter and landed in granulated-sugar dough. Managed to scrape all dough off; however, is possible that impact of highly-respected cookbook at critical mixing stage damaged dough's self-esteem. Further experiments needed, possibly tossing at dough Joy of Cooking, Baking with Julia, Maida Heatter's Cookies, etc.