Equating driver diameter with it's SQ properties isn't a good idea...any sized sub can be integrated into an SQ system...I can name plenty of people who have used 15" drivers or larger in extremely successful SQ vehicles.

If you were really going for SQ, multiple drivers is the way to go...the more cone area you have, the less excursion you need to create a tone with the same SPL....less excursion = less distortion...then again this is coming from a guy with an IDMAX

If it was me...2 12's