The 18 should theoretically be louder due to more cone area. But it depends on the space you're working with, how efficient the 18 is, etc. If you're looking at a trunk setup, the 12's would be louder due to the fact that you could fire them into the cab sealed off. The 18 would barely fit firing into the trunk lid.
You say "under the same circumstances blah blah blah", but these circumstances truly do matter. Hope this helped.