I think the 12's will hit a harder from ~40hz+ but the 18 will own everything below that - JMO though. I'd personally go with the 18 because I love my lows.
yeah i can fit 2 15s in car but i am not about to change my subs to 2 15s. i found one on ebay for $300 plus shipping for 1 ssi 18 so i was just curious.