The first variable you want to look into is, what type of sound do you desire to get our of your subwoofers. If your looking for the deepest sound inside the car that is crisp and quick as possible, you want a sealed box. Sealed boxes are just a box that is sealed and has no air gaps. These have been around for ages, and are very reputable to go with.
If you want the bass to be heard from other people outside of your car and as far away as possible, than a vented box is more your style. Vented boxes are somewhat new technology and are said to be double the loudness in a lot of instances. Your subwoofer will have special specifications of what frequencies the port or vent should be tuned to.
The box can make or break how loud your system is, so it's a huge deal. I have seen the cheapest car audio equipment sound ten times louder than a guy that spend thousands because the box was better designed. Look into what box best suits you and your subwoofer. You may regret it, if you don't make the correct choice from the get go.
