are 12 inch subs better than 15 inch subs in a 2000 dodge intrepid?

I have a 2000 dodge intrepid with 2 15 inch shockwave subs but a friend is telling me that i should get 12 inch subs because they will have more space to breath then the 15 inch subs but i still think the 15 inch subs will be louder which one is the best choice