with the right power them 18s will knock. the 12s will be nice but them 18s outdo the cone area by about 282 square inches...thats basically DOUBLE the cone area of the 12s.
why not hit the happy median with some 15s?
speakers can handle any ohm load...is the amp that takes a beating from a low ohm load. you can run a speaker at 8 ohm 2 ohm 6 ohm .5 ohm wont hurt it as long as the amp is stable at that set ohm load.
with speakers though the lower the ohm load the more THD that will be produced. it SOMETIMES makes an audible difference depending on the quality of the amp. Some quality amps can sound better at 2 ohm than a crappy amp at 8 ohm.
not bad....tweak the box and add more power and you can hit much harder. i hit a 141.7 with 600 watts and 2 12s...(im not trying to down you one bit by saying that) im saying you can hit a pretty good number with little power and a good box