It's completely install dependent. If you have the volume for a 15" then do that, if you only have enough room for a 12", then do that.
I'd much rather have a smaller sub in an ideal box than a bigger sub in too small of a box.
Answer your question?
Don't let anyone tell you " 10s are more punchy, 18s are too sloppy" etc.. Lol those factors are completely install dependent. You could hear both in the back of my car and have no idea what size subs are in there.