2 subwoofers or 1 bigger one?

When it comes to cone area, one 18" sub has more than 2 12s which means that in a proper box and plenty of power it will sound louder. There are a few disadvantages. First, the enclosure for this sub will be huge, specially if going with a ported box. Second, it will need plenty of power. Third, if you care about sound quality over hard hitting bass, the 2 12s is the best option.