the truth about speakers (somebody needs to make a sticky for this... as these questions keep coming up, i'd write it if permitted)
firstly, ALL these woofers (Even 18's) will produce up to 500hz or higher before they cones break up into nodes (can't move fast enough to move as "one" and begin to ripple) obviously nobody wants their sub to go to 500 hz so this isn't even a consideration.
these are the important ones:
peak throw! how many MM of peak throw does it have? more is better (cone can swing further without bottoming out) remember that a 12" woofer will have to move further to move the same amount of air as an 18. if you want to compare, assume 1" or the speaker doesn't move (surround, mounting etc) and do pie for area.
SPL at 1watt! this tells you how loud it will play with one watt of power.. this is a universal figure which you can compare speakers. the louder it plays with 1 watt, the less power you will need to run the speaker. remember that in order to run a speaker 3db's louder, you have to double the input power. so a speaker with 93 DB at 1 watt would take 2 watts to play 96 db, 4 watts for 99db, 8 watts for 102db, 16w for 105db etc. a powerful speaker is worthless if it is horribly inefficient.
resonant frequency!! to me this is the most important factor, this factor determines how "deep" the speaker will produce sound.. whatever the resonant frequency is, the speakers output (in a sealed box) will decline at 12 db per octive from that point downward (an octive is a doubling, or half-ing of frequency, for instance, 20 hz to 40hz is one octive) so if you're speaker has a resonant frequency of 60hz, and you want it to produce bass at 30hz (or you're low pass filter is at 60hz), it's going to be a bad choice. the speakers usable response starts at the resonant frequency, and goes UP from there. a speaker in a ported box will drop off faster than 12db/octive below the resonant.
also keep these rules in mind:
every increase of 3 decibels requires DOUBLE the wattage.
doubling air displaced (twice as many speakers) will raise the SPL by 6 decibels.
it takes about a 10 decibel increase in sound to appear "twice as loud" to our ears.
some general things i'd look for in a woofer would be: big magnet, lots of throw, low resonant frequency. personally i like bigger woofers as it takes fewer of them to displace the same amount of air (they are more efficient)
also i'd consider using a home woofer for you're car, as most "car audio" woofers just have a higher resonant frequency (higher resonant frequency speakers play louder with the same wattage, at the expense of bass response) this is why some car audio systems play really loud, but have "one note" bass (their resonant frequency is at about the same point as the crossover frequency, so they only really play one pitch loud)