Moving air is good, the more you can move, the better for a number of reasons.
Compared to 1 driver, 2 drivers operating at the same output as the 1 driver offer 1/2 of the excursion and 1/4 of the distortion (see jbl comparisons of 2 15" drivers in a theater speaker as opposed to 1 15"). Larger drivers move more air than small ones so they don't have to move / work as hard for same output.
Doubling output in volume is a biggie. You need to double your drivers to gain 6 more db. Common belief is 8 to 10db means twice the volume to your ears. So 100 watts to 200 watts is only 3db, you probably wouldn't even hear a volume difference. So say you have a 12" with 90db spl that takes 100 watts (don't plan on more because usually heat builds causing compression, most drivers won't to your ears play much louder with 500 watts than 100 watts) so you have 110db (not including boost in car), 1 more driver with same power, you now have 116db. Adding 1 more driver will not add 6 more db, but 2 drivers will add 6db. Double the box area, double the amplifier power, for not quite twice as loud.
What is better 2 18"s or 4 12"s? I've heard a friend of a friend using 12 12" all solobarik for >150db, granted it takes up most of the bed in his truck with a topper.
Hit harder, if you mean volume it depends of the sensitivity of the drivers. If you mean hitting and not droning on after the note, alignment is the key.
I use 4 18's in push pull (not isobarik) in a sbb4 (qtc.577 sealed size but ported to f9 of 27hz with a high pass q of 2 6db boost at tuning for -3db @27hz) in a house. 18ft3 per box of 2 drivers. The guy that built the boxes scoffed at them "sealeds are cleaner than porteds". Once he listened he said "I never expected 18s to be that quick" What he didn't know is a ported has the same group delay at twice the ported frequency. So tuning to 27hz means the drum kick at 63hz sounds as tight as the best transient response sealed box with a qtc of .577 alignment. Yes there is delay as you go lower, but the frequencies that low are mostly sine waves and longer droning explosions not as transient as music.
so it's up to you. 18s usually have more sensitivity. Some say 18s can't go very high (100hz), but that is not true either.
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