More or bigger subs?

ok which would be better to get 2 12" orion hcca subwoofers in a 5 cu.ft box or to get a 18" fi btl subwoofer in a 5 cu.ft box. I just wanted to know cuz i was told a 18" sub could displace more air than 2 12's. im trying to get loud car shaking b. Or should i just jam alot of 10" subwoofers in the trunk..?