First, consider the application. Speakers meant for a less than critical application, such as a bathroom, probably do not need the top of the line. But a quality home theatre system would be deserving of higher end subwoofers.
Second, consider your budget. The better the speakers, the more they are going to cost.
Finally, don’t forget to take into account your expectations. If you’re a very serious and very critical listener, then go for the best you can afford. But if you’re a less critical listener, you may be satisfied with a less costly system.