1 15 re sx in a 3.944cuft ported box tuned to 28hz or 1 12 re sx in a 3.521cuft box or a 2.686cuft box tuned to 28hz both will b powered wit a ed nine 1 what 1 will b the best in loudness an best bass
... the 15, it has a huge cone area advantage. I don't understand why you're asking this its obvious the 15 will get louder and probably sound very similar in a vented enclosure to the 12 SQ wise.
The 15's probably the sub for you, it's going to be very similar to the 12 with more output and it seems that's what you're after. Good luck with the setup.
What type of music do you listen to? The 12s would be better if your more into rock but if you listen to rap or other types of music with deep/varied bass the 15s are the way to go. More air flow = more bass. The 12s frequency response would be a bit quicker than the 15s though.