The three factors that can really affect the volume of your sound system are sensitivity, enclosure type, and available power.

Sensitivity or efficiency is the woofers ability to convert power into sound. The higher the sensitivity the more effectively it converts power into volume; so a sub with less power but a higher sensitivity could be louder than its more powerful counterpart with a lower sensitivity.

The type of enclosure can drastically change the type of sound that your sub puts out. Sealed enclosures produce the deepest most precise sound and require more power but ported and bandpass enclosures are louder with bandpass being the loudest.

When you choose a sub you will also need to choose an amplifier that can provide the sub with all of the power that it needs and generally that high power the sub the more sound it will produce unless one has a much higher sensitivity than the other. You need to match these numbers based on the subs RMS power not its Peak power.