If youve got the space a 15" will be louder than the 12... other than that there really isnt a difference. Now if you are talking a ported Q12 vsa a sealed 15" there are some differences. Sealed will sound more natural overall vs ported. Sealed 15 will be a little smaller as well vs the ported 12. Sealed 15 will be louder up higher in the 70+ Hz range and about 1/3-1/2 octave below ported tuning. Ported sub will be louder near tuning...
If youve got the space a 15" will be louder than the 12... other than that there really isnt a difference. Now if you are talking a ported Q12 vsa a sealed 15" there are some differences. Sealed will sound more natural overall vs ported. Sealed 15 will be a little smaller as well vs the ported 12. Sealed 15 will be louder up higher in the 70+ Hz range and about 1/3-1/2 octave below ported tuning. Ported sub will be louder near tuning...