IT REALLY ALL DEPENDS ON THE TYPE OF MUSIC YOU LISTEN TO, AND THE QUALITY OF THE EQUIPMENT YOU ARE LOOKING INTO PURCHASING. IF YOU HAD A SET OF 10, 12, AND 15 IN THEIR RIGHT ENCLOSURES WITH THE RIGHT AMOUNT OF POWER FOR EACH YOU WOULD DEFINITELY NOTICE A DIFFERENCE IN THE SOUND. THE 15 WILL GO LOWER(MEANING THEY WILL PLAY LOWER FREQUENCIES, THAT LOW RUMBLY BASS) THAN THE 12 OR 10, AND THE SAME WITH THE 12 TO 10. IN THE RIGHT ENCLOSURE AND WITH THE RIGHT AMP YOU WOULD PROBABLY BE HAPPY WITH A SET OF 10, BUT IF YOU LIKE TO LISTEN TO REAL BASS MUSIC I WOULD RECOMMEND YOU GO WITH THE 12 OR 15. THEY HANDLE MORE POWER(ASSUMING YOU ARE COMPARING EQUAL SPEAKERS-EX. ALL KICKER SOLOBARIC L7), AND THEREFORE PROVIDE YOU WITH THE ABILITY TO USE A BIGGER AMP TO GET YOU MORE PUNCH(ASSUMING FAIRLY CLOSE SPL RATINGS BETWEEN THE SUBS). IF YOU CAN AFFORD TO GO WITH A GOOD SET OF 12 OR 15, AND CAN AFFORD TO GET THE RIGHT AMP TO POWER THEM THEN I WOULD GO FOR IT AND NOT LOOK BACK. I KNOW TOO MANY PEOPLE WHO GO AND BUY A SET OF 10 AND THEN DECIDE THEY WANT MORE BASS AND HAVE TO GET NEW SUBS. I WOULD JUST DO IT RIGHT THE FIRST TIME. BESIDES, NO MATTER WHAT YOU PUT IN THE TRUNK(BE IT 10,12, 15) , IT WILL BE RENDERED VIRTUALLY USELESS BECAUSE THEY AREN'T THAT BIG TO BEGIN WITH.
HOPE THIS HELPS.