No question. Get the bigger sub. Remember that it takes a doubling in power to get +3 dB. Also, look at the subwoofer efficiency (typically measured in dB/W). In almost every case, the 15" will have a higher efficiency than a 12". Also, the increased surface area means that you move more air, making the bass more powerful.
That is very much an oversimplification.
Double the power doesn't really equal a 3db gain after power compression and heat losses.
Yes a 15 will often have a higher efficiency, but the reality is subwoofer efficiency a joke. It's measured at a frequency that they don't even play.
Surface area doesn't mean anything. Effective displacement matters. A 15 will have roughly the equivalent displacement of 2 10" or 1.5 12" subs of equal Xmax.
I'm an advocate for putting as much cone area as you can into the space you have available. Adding power is pretty easy, re-building an enclosure not so much.
Matt