I think Wooch summarized it very well.
Generally, sealed subs are said to be more "musical" than ported subs (at the expense of higher output, and some low extension). If you're buying the sub for music purposes only, I'd recommend going that route. There's several technical reasons why this is...I won't get into those now.

If you use a sub for both home theater and music (many of us do) than it's a job of balancing trade offs. You can get super low bass extension in a ported design (5 Hz or so lower) and much better power handling and output ability - this is great for home theater. To get the same performance from a sealed sub, you have to spend more money, or buy a much bigger sub (probably a 15" woofer).

All this isn't to say that a ported sub won't sound good for music, or a sealed sub good for home theater, just that each have their own design advantages

Depending on your budget, sealed subs might not be much of an option though. If we knew roughly how much you were budgeting for this, we could probably make a few recommendations for some of the more popular subs here.