Personally I have found that it depends on the mood at the time.

Myself, I am inclined to be dom during the day (at work) but find it to be a relaxing relief to be a sub when I get home. With my partner we enjoy the ability to assume a given role (and we both tend toward the dom side of things), and with us both being very strong-willed personalities it works well in the relationship to give in at times.

Somewhat perversely, we have times where we both want to be the sub or the dom, but that simply adds excitement while we determine who's going to take on which role that day, or that hour, or that moment.