you are refering to the motor being the same, this is only true if they are the same size but they are not, the 12 l7 takes 750 watts, the 15" cvx takes 1000 watts, the question is actually fairly valid...
take a 12 cvx, and a 12" L7, due to the nearly identical motors the only advantage the square really has over the round design is increased effective piston area makeing it louder, but in most cases it will have less sound quality and less Xmax to achieve its max level due to many factors involving the square cone..
now moving up to the 15 takes the surface area advantage away, and is even better because of a higher xmax, and better SQ of the round sub... so yea in purty much every application the 15 cvx will be better than the 12 L7...
and i personally think a 15" round sub looks like it wants to tear a 12" square apart