What is important with a sub is that which is important with any other speaker, really. You should be concerned with frequency response going down low enough (for movies I recommend shooting for 20 Hz or less if you can manage it), high enough (that is, high enough to mesh with the low end of your other speakers), and reasonably 'flat' within that range. Then of course there's the actual volume it can achieve for you, which you can tease out from various spreadsheets and online reviews. Whatever combination of driver size, enclosure size/design and wattage and such the manufacturer uses to get there is a bit less important, as there are several different compromises from which to choose in order to get the desired effect.
Of course, some designs perform better than others in terms of clean sound reproduction, but I would suggest filtering first by capability and then weeding out the ones that people commonly complain have 'chuffing' or other undesirable sounds. You can also weed out the ones that cost too danged much for your budget.
Many people think that sealed subs are better for music, whereas ported subs are better for movies. That being said, my ported sub sounds just fine when I listen to music, and a sealed design that plays the sounds you need from a movie soundtrack competently should make you smile, too. Ports are a way to get a bit more bass out of a given design, at the expense of some volume higher up the frequency range. A few subs are designed to allow you to run them either way according to your personal preference.