Considering you're comparing the same subs, both in correctly speced enclosures, I'd say the 15 would. It's seeing more power. Going loud is going to depend on the woofer itself, the enclosure and the amount of power going to it.
yea but wouldnt it still only be a total of 2000 rms apposed to 2500-3000 so wouldnt the 15 be louder still with more total power or does the cone area make a really big difference?
It'd be a close call either way. Maybe on a mic there may be a difference but to the ear, I would guess not much at all. What kind of car/truck would it be going in?
figure the surface area of the combined 12" drivers, and that of the 15" then look at the Xmax (excursion) of the combined 12" versus the 15" then compare the sensitivity of each, and the power applied to each. It's not really an easy comparison. Especially if you plan to use different enclosures.
Are you only concerned with SPL? Are you also considering low end extension? How much room each would take up? Where the output will peak for each?