Huge differnce. I have both , and its a huge difference. Just for the record, other 18's might get slugish, but not a x.x.x. 18" .SQ and SPL is awesome. Even the SX ,wich is more of an SPL sub, has great SQ.A x.x.x. has better SQ than a SX but the SX is noticebly louder.To answer your question, both 15 and 18" sound awesome with the 18" being a lot louder.