Hmm...
While maximum output and extension is important, you can throw both of them out the window if the signal is a distorted mess that sounds like a dump truck in low gear.
Tight, high output SPL will be more effective in almost all cases than extension and aesthetics. Don't misunderstand me, I love to feel the bass as well as hear it, even more so when the sub is going to be used predominantly in a home theater setting. However, low extension is irrelevant if its not well controlled and isn't a good representation of the original signal.
Size and aesthetics also come into play and is typically dependent on the setting or application of the sub itself. However, this it generally the last of the aforementioned criteria I suggest and adhere to that a subwoofer purchase is based on. -TD