Bigger is not always better. More cone area more plus smaller speaker will react and move sound quicker than a larger one. 2 12's as a general rule will ALWAYS be loud as f*ck if hooked up in the right setup.
I like the one woofer idea and given the right setup and power it could be as loud as two 12's. Its all about the install mang. Thought most you peeps knew that by now.