That's a tough one. I love them both. Gianna seems to be more agressive in her movies(which I find to be really hot), but I'm gonna have to go with Carmella on this one.
Carmella isn't that high on my list by any means but I would go with her, especially if I'm choosing just based on looks.
I know there are people here that like her and I don't have a problem with that or anything, but in my opinion Gianna Michaels might very well be the most overrated person in the history of porn.