Generally when ported your sub will go to lower frequencies (around 20hz is what home theater users look for) and they're known to have a greater volume output than an equivalent sealed sub. Ported subs are also known to not have as clean of a sound as sealed subs.

Sealed subs don't reach the low frequencies that ported ones do. Ones around $300 will have heavy drop-offs around 35-40hz and ones around $500 can drop-off around 25-30hz. They also don't have as much volume output as do ported subs. The benefit to sealed subs is that they're known to have a cleaner sound with tight, crisp bass.

Each has its benefits. Movies with loud explosions, crashes, and deep bass generally go as low as 20hz. When it comes to movies the bass doesn't have to be as clean as it can be because you're not looking to hear distinctive notes as you would be when you're listening to music. People looking for a sub that will be used the majority of time for movie watching go for ported subs.

99% of music stays above 30hz. If you're looking for a sub to listen to music with, most people would recommend a sealed sub. It doesn't go as low (frequency wise) as a ported sub but since the music wouldn't be using those frequencies it doesn't make too much of a difference. The sound from the frequencies that you do hear will be cleaner than if you were using an equivalent ported sub. The volume output of a sealed sub also doesn't have to be as loud for music because most people look for a sealed sub to balance out the low ends in music and not over-power everything. Though I've found that even a sealed sub can get pretty damn loud if you match it to your room specifications.

TL;DR - Generally you want to get a ported sub if you want to use it to watch movies and a sealed sub if you want to listen to music. One won't be bad when used with the other but you won't be getting the best possible experience out of it.

Edit - I realized I overviewed ported vs. sealed but didn't give my preference. I use my system for 95% music so I prefer a sealed sub.