Besides choosing between the loads of car sub woofer manufacturers you will want to figure out the specs of the sub woofer you are looking for, this will ultimately give you the correct sub woofer for your listening needs and allow you to find the best deal out there. There are distinct advantages and disadvantages to each sub that you choose and I will outline some.
1. Size- The size of your sub woofer is a major factor to consider, for a few different reasons. If you have a smaller vehicle you will be unable to fit a gigantic woofer into the trunk but you also do not want to regret getting a woofer that is too small for your needs. After determining the amount of space you are able to work with you can look at the sound advantages and disadvantages different sizes offer. The larger the woofer size you choose the better lower end you will end up with and can hit notes you normally wouldn't get out of your typical system. One disadvantage to large woofers in terms of sound is their transient response is not nearly as able to handle quick changes in tone as well, this sometimes results in muddling to the tuned ear. A good size that combines a good transient response while still hitting pretty low and can be VERY loud is the 12 in woofer. You will find this in many many vehicles due to the combined advantages it has over other sizes.
2. Power Handling- If you have a budget this is something you will want to watch because it will affect how much you want to spend on your sub woofer and amp. There is no point in buying a sub that can handle twice as much power as the amp you bought, it will hardly increase the life of your sub woofer and will do an injustice to that sub. Obviously the higher power you get the louder your sub can go, but with that increased power you will also need to be able to cope with the drain on your cars charging and battery systems. If you go big on the power of your sub you will definitely need a capacitor and depending on how hard you need to bump, an upgraded alternator might be in order.
3. Voice Coils and Impedance - This will impact how much your amplifier outputs to your sub woofer. The lower the impedance on the woofer the more power that the amplifier is able to feed to it. Again many of the specifications of the sub woofer you choose will have to coincide with an amplifier choice. There is a certain amount of resistance that the amplifier will be able to safely handle without risk of losing the amp you just spent a fortune on. Your run of the mill amplifier will typically have a lower limit of 2 ohms, and the higher end amps will go as low as 1 ohm for their max output. You will want to pay attention to the sub you choose and the ohms that it has because you may not be getting the optimal output from your amp if your ohms are higher than the amps lower impedance parameter.
I have linked the voice coil and impedance for one reason, the type of voice coil set up you choose will effect how you are able to wire your sub woofer and change the impedance. Your two choices of voice coils would be a Single Voice Coil (SVC) or a Dual Voice Coil (DVC), and in terms of loudness neither will be better than the other. If you already know the lower limit of your amplifier is 2 ohms and you don't want to worry about the wiring of your sub it would be easiest to buy an SVC sub with a 2 ohm impedance. If you plan on ever changing or upgrading your system than you may want get a DVC sub because you will have more options in terms of your impedance options. If you buy a 4 ohm DVC you have the option of using it as a 4 ohm sub if you wire it in series and if you wire the voice coils in parallel then you would have a 2 ohm load instead. I will get more into wiring later on in the blog.
There are many more specifications that you can choose from but those are typically for much more advanced users that are in need of very certain things out of their sub woofers.