It's not just about cone area. It's a plethora of things. IE xmax, rms, box, the car etc etc they all play a part. As its been said numerous of times and and many more times to come..test test test. Yes, two 15s means more cone area, but it doesn't mean it will be 'louder' or better.
If it was just about cone area, people wouldn't be hitting 160s up with one sub. It's all in what your own preference is.
Back on topic though, say if you are looking at Kicker L7s, then I would choose two 15's, but if you're looking at two Pyle 15's vs 3 T3 T2000 12s..I'd choose the 12s lol.