I prefer
Brenna only because
Carly has been done and overdone since the 80s. I thought Carly/Carli/Carleigh/Carley used to be cute until I met so many people around my age with this name.
I class
Carly and
Brenna as the same type of name, though. The Brennas born today are the Carlys of the 80s. Both names try to seem unusual and strike up images of blonde suburban cheerleaders in my mind.