I don't know if anyone else has said so yet, but the price would be the first and possibly most important influencing factor when choosing a subwoofer, for me at least.
Let's say I have in mind to spend $800.00 on a subwoofer. Well, I'm going to immediately eliminate anything that costs more than a $1000.00 no matter how good they look, how accurate and ear pleasing and earth shattering the sound.
Everyone has a budget, and from within that budget you then consider a handful of models with good word of mouth (from both users and professional reviewers) and then seek those models out for a live demonstration where possible.
Excluding the "other" option, I would be most concerned with the overall sound abilities as in does it work for both movies and music and how will it do on both low, medium, and loud volumes in the space I'd be using it.